So I've recently seen PbP talk and my first thought was "wow cool! a way to play dnd when you can't find players irl!" but it got me thinking. HOW does it work? I think I can sum up my confusions in a few points...:

1. How is "reacting" managed? By this i mean, How do players "take turns" during RP moments? I keep having this thought of 2 players typing opposing responses to a situation at the same time. How is this resolved? How would one player react to something that another player did in the middle of their typed description? For example:

Player 1 (introducing party to NPC): My name is steve and this here is my friend bob, we're adventurers from neverwinter and we're looking for work"

If Player 2 (Bob) was playing a shady rogue and wanted his name secret, at a table, he could simply interupt player 1 like "....and this here is my friend B-..." "*I cover his mouth* JIM! my names JIM!" But in PbP, player 2 doesn't know what player 1 said until his whole post is made. How is this situation handled? 

2. Etiquette. How do you manage having multiple people responding to the DM? Do you just  think "Well I responded last time, lets let someone else say something"? at a table, one person can start a conversation and others can build off of it, but when posting, I think it would be weird to post like.. half a sentence? Example:

DM as a king asks a question to the party. The way I see it, a few things will happen. A) Everyone types their own answers and posts at the same time. This means (in game) they characters either all shouted at the same time, or they all responded back to back with their own individual answers while the king waited. either seems weird to me. B) one guy answers and the rest of the party is silent. This is where I'm wondering if people will "be cool" and let others have a shot. C) no one says anything because they're all waiting for someone else to say something. D) One player says something, a second player has follow-up, but the king has already responded to the first guy meaning the second player's response didn't happen??

3. Timing and Amount of description. This goes with both the other points. Is it just first-come-first-serve when it comes to doing stuff? meaning, if your posts is first, your action is first (outside of combat)? And when you describe what your character is doing, how far can you take it? I feel like if its too little (Example "I walk to the corner of the room") Then the game would take forever. But if its too much (Example: I sneak over to the corner of the room, toss a copper to distract the barkeep and try to steal his keys while the party isn't looking") then there will have to be backtracking such as another player saying "did I notice him leaving by himself? I would have followed" or "DM: There was actually an explosive trap in the corner so you didn't get that far"?? 

Maybe I'm overthinking it but I'm having a really hard time trying to think how this runs smoothly lol

Thanks!

It's slightly group and campaign dependent. Some groups may supplement pure PbP with bursts of chat for instance, if everyone can set some time aside at the same time - this can be practical for combat encounters for instance, given that those usually consist of short and fast responses. Some campaigns might be more freeform, with extensive roleplay among the players without needing real DM adjudication - the actual campaign-related posting still needs care taken regarding the flow, but everything else is just a couple of players doing their thing. What I'm saying is, the "style" of your posting can vary even in a single campaign, depending on context.

As far as the "real" play-by-post is concerned, groups will have their own dynamics and flow. Some might edit their posts fairly significantly to remove inconsistencies, other might prefer limiting edits to a minimum (either way, if you make an edit you should indicate why, so the others don't just see an edit indicator but don't know what you did). Some will have longer posts on average than others. You'll have to feel it out a bit.

Point-by-point: 

1) this can be handled through editing, if necessary. That said, it's always worth at least considering just going with it. This sort of thing happens in face-to-face or real-time chat as well, after all. Sometimes it's just better to accept what other players do or say - you can't control everything perfectly, sometimes you'll just take your lumps and deal with it.

2) typically everyone makes one response to a DM's post, then the DM adjudicates what happens in a new post, everyone posts once in response to that, and so on. This doesn't have to be a hard rule, if you see another player's post before the DM's responded and you think it's ok to post something in reaction to that that can be perfectly fine for instance. Some groups may also have rules about posting frequency, so they can move on if someone doesn't reply within 24 hours or whatever time limit they set. As an aside, if you know you won't be able to post for a while it's good manners to notify the DM and other players so they can cover for you or do some stuff you don't have to be involved in until you get back.

3) this can vary quite a lot by group. Personally what I like to do is have a "this is what I'd like to do" part in a post, that describes how your character is going to react to the DM's cues, and a second part that describes what my character my character did and how a bit more after the DM has adjudicated the last flurry of posts and given their next cues. 

For instance, the DM describes the group entering a cave, a rough oval about 70 ft deep and half as wide , with a pool of water that's likely connected to the ocean in the middle, some moonlight filtering down through a small opening in the ceiling, and a stone structure roughly the size of a large table, maybe 5 by 15 feet, that looks man-made on the other side of the pool. 
One player says they want to light a torch of the moonlight isn't enough to make a thorough search possible and go along the walls of the cave first to make sure nobody's hiding. Another says they want to check the pool. The third wants to check for traps and when he sees the first two's posts, that they want to caution them before going inside. The last player says they want to ready an arrow and shoot if anything jumps out at one of the others investigating the cave. They'll likely be a bit more descriptive than I'm being here (that goes for the entirety of this example), but the point is that everyone announces their intentions.
The DM steps in again and explains that a torch isn't needed to see the entire cave, but it might help with small cubby holes or areas in deeper shadow - player 1 can proceed and should make a Wis/Perception check; the pool itself is well-lit by the moonlight from above and maybe from some unknown light source under the water as well - it's not possible to really see down into the water from the entrance of the cave though, player 2 will have to approach if they want to get a good look; player 3's caution is noted (and the other players can choose to heed it or not) and they can make an Int/Investigation or a Wis/Perception check to try and find traps, their choice; player 4 can nock an arrow and prepare to fire on any unexpected guests popping up.
In their next post, the players can then describe in a bit more detail how they search the cave, approach the pool, check for traps and cover their team mates respectively, as well as deal with any checks the DM told them to make; in that same post, but in a second part, they can then announce what they intend to do next. Player 1 says that if they don't find anyone, they'll move over to the table-like structure next, player 2 wants to see if there is a light under water and if so what it's coming from, player 3 wants to see the table as well and player 4, if nobody gets attacked, says they'll try to rig up some kind of trap just outside the cave so they'll have warning if someone comes up behind them while they are inside.
Then it goes back to the DM again, and so on and on.

You'll find a balance between announcing enough to keep things moving along and not announcing too much for the DM and other players to react to soon enough, I expect. Even if you can't interact in real time, it's really not fundamentally different from face-to-face play: you won't go into your character's actions far enough that you'd be skipping DM adjudication at the table either (and if you did, the DM would step in and you'd correct yourself after that). It's likely going to take a bit of trial and error at first, but that's ok. Face to face groups have to get a flow going if it's a new group as well, and that'll also take a bit of time and players adjusting to each other. 

There’s a whole section here for just this - go and pick one of the longer threads and start reading, you’ll see how the interactions and such go and if you think you’d find it interesting...

Thanks for the responses! I know theres a PbP section, but I saw those were specific games. I didn't want to spend time digging through and comparing other's games. I just wanted to know if there was kinda a general feel/informal rules on them!

Is it safe to assume that these types of games work better with fewer players? It seems like a lot to deal with ANY more than 3-4 total people (including DM!) I play in 2 ongoing campaigns with 6 players + 1 DM and the amount of Back-N-Forth conversation we have I don't feel would be possible with that many people when you're waiting your turn to respond

Quote from LeBattery >>

Is it safe to assume that these types of games work better with fewer players? It seems like a lot to deal with ANY more than 3-4 total people (including DM!) I play in 2 ongoing campaigns with 6 players + 1 DM and the amount of Back-N-Forth conversation we have I don't feel would be possible with that many people when you're waiting your turn to respond

First off, don't expect things to move along fast regardless of the number of players. If everyone involved is online and active at the same time and for a good while every day, you can probably keep a decent pace. If people's schedules vary a lot though, it's not unlikely you won't post more than a couple of times per day. So, manage your expectations.

Second, I was involved in a more or less sandbox type play by post game for several years a long time ago. We had upwards of 30 active players at one point, and a team of DMs that coordinated their tasks. That worked great too. The number of players doesn't matter as much as their availability - though the more there are, the bigger the chance one or more can only post sporadically - and how much effort they put into it.