Okay, simple premise, but I hope this will make way for a larger discussion. 

Thieves Guilds and Assassins Guilds are different. One is a bunch of thieves that work in an underground network to transport and sell stolen and/or illegal goods, while the other is filled with hitmen/hitwomen that can be hired to do the dirty work of those with the means to pay for it. A great example of the differences between the two are the Dark Brotherhood and the Thieves Guild from the Elder Scrolls video game series. There is a bit of thematic overlap, as they tend to be rogue-ish characters adept in sneaking, persuasion, and fighting, but there are big differences and can both exist separately in a fantasy setting. 

This thread is to share what Thieves Guilds and Assassins Guilds you have included/(or came across if you're a player) in your campaigns and worlds, and if you have both types, how you uniquely differentiate them. My world's examples will be in the spoiler below:

So, what are your world's versions of these common guilds? I'm excited to hear them. 

There's nothing really special about my Thieves' Guild, although I have to update the org chart after much of the city was burned down in a battle.  The undisputed leader now is a human archfey warlock named Easy Calloway, who had previously been the Guild's "Minister of Information."  The Guild had, until recently, been engaged in a long gang war with an organization called The Yellow Sign.  They had them on the ropes when the battle happened, but in the aftermath, neither was able to finish the other off.  The loathsome Prince Waleran of Crum, who ascended to the Regency after Duke Peregrine's war blimp was shot down by dragons, seems strangely incorruptible after Easy had spent a great deal of time grooming him as an ally (The Yellow Sign has had his brain replaced with an intellect devourer).  Meanwhile, in the city's weakened state, other groups are moving in, including the Cult of Asmodeus, which has  First Minister Lionel Turnbridge as its political patron.  So now, the Guild, which lost most of its soldiers in the dragon attack, is trying to negotiate an alliance with the hobgoblin gangs that run the Redgate Stockyards and their mysterious enforcer - Medva Solenya, the Pickle Witch.

Barring player intervention, the Guild should be back on their feet soon enough.  The Yellow Sign will soon have all the human brains it needs for its sinister purposes and the Cult of Asmodeus is happy to play the long game, controlling vice in the new developments north of the river while expanding their legitimate holdings in the city proper.

It's peripheral, as we don't have any assassins and assassination attempts - either executing or preventing - aren't really in my parties' wheelhouses at this time, so really it's only in place if the PCs ever find a thread to pull, but The Table from the John Wick universe, I've completely ripped off for my game if I need to.

Thieves guilds in my game aren't anywhere near as over the top orchestrated.  Basically they're racketeering enterprises.  Some dominate a geography completely (sometimes just the underworld, other times more publicly recognized "community leaders" if not governors.  Others co exist among other groups each with its own "piece of the action."  And in other places differing groups are at differing stages of cold or hot war with each other.  And it's not a static situation and can change over time (as the game needs).  

Real life orchestrated murder is ugly and almost always stupid stuff and not stuff I'd want my D&D game to dwell in, so in my game I prefer to go for something much more absurd and dignified.  Other areas of crime, particularly organized crime can be a bit more interesting, so the way I populate the rest of my criminal world I lean more into realism.  

Quote from Vince_Snetterton>>

If your world consists of separate guilds, so be it. Given that so many of the traits for both types of NPC/players are the same, mine are bundled into one.

You're saying this in a way that comes across as judgmental. If you don't mean that, sorry for the misunderstanding, but I just wanted to point out what you seem to be saying. 

The Guiding Hand Social Club is the name, stolen from a seriously hard core game called Eve Online that many of the modern D&D players would run screaming from.

This is the excerpt I created for my players:

Througout the history of humans living in Medina, The Guiding Hand Social Club (GHSC) has been part of the darker chapters in that history. The GHSC is the most powerful thieving guild in Medina, with powerbases in all the major cities, extending even to the major Elven cities on the Gold Coast. Though the GHSC is not the largest by membership, it is by far the most ambitious and accomplished of the thieving guilds. Their assassins and master theives have been implicated in the most complicated, long-running, and sometimes lethal plots throughout Human history, and if the rumours are true, recent Elven history.

While they are almost universally hated and feared, it is common knowledge that they are hired by many of the human nobility to work their crafts against other noble houses, and the GHSC is also known to provide various services for free in the ever-ending battles against the Orcs, whom the GHSC hate, because the Orcs have destroyed potential income streams of human wealth.

There is no known members of the Guild operating in Hillsgaard, but that means little, as their members make a point of maintaining very low profiles. Anonymity is a key to their survival, and success. In the past, when the economy was much stronger, there was little doubt they operated in Hillsgaard, and probably Smithy. They most certainly have a presence in any city of size and wealth.

Thanks for sharing your example, though :)

The Shadow League is an underground criminal organization that focussed on smuggling, but the will also steal and kill. They are a hidden deep secret, almost like a myth. Think Keyser Soze from usual suspects, someone knows someone who once did a job for someone who said they where hired by the league. 

However the league has a code, it won’t kill the innocent, it won’t steal from those who are poor or weak, it won’t take part in slavery or the transport of slaves. It has assassins and thieves and smugglers and forgers and everything else you can think of. 

It is also the organization that keeps the peace and stability of the land, it’s founder and ruler is a silver dragon, who centuries ago realised in a vision that if the fracture and warfare between nations continued it would lead to the fracture of the world, so it formed 2 organizations. A magic users academy and repository of knowledge. A place for magic users to learn, to understand and to be safe in there application of magic. A place where advisors could be trained and sent out into the world to ensure the nations entered into alliances, or ensured the right rulers won the wars. 

But the dragon relalised that sometimes a scalpel would be required to remove a problem, so the league was created, an underground shadow organization who would place agents in every major population center, who would infiltrate governments from below, blackmail, extort and, where necessary, kill anyone who threatened the stability of the world. But they needed to be funded. The academy could openly source resources from the nations who’s magic users it trained, but the league, they needed to gain there own funds and so, for the greater good, they began running wider criminal schemes designed to raise and maintain the funds needed to keep the world safe. 

My other favorite criminal organization from fiction, and one I will one day work into a DnD campaign. The Kolat from legend of the 5 rings. An organization of mortals that funded itself through crime and managed to manipulate and control the gods themselves brining about the end of the godly rule of an entire nation. 

I'm still working on this world, but the basic premise is that a young boy stumbles upon an ancient artifact that perfectly grants one wish, and he wishes to be King of the Nation he lives in. Which is all well and good for him, but he's still a literal child and easily manipulated, so the Thieve's Guild manages to completely take over the Town Guard, and now they're essentially a giant, government-funded Organized Crime group. This is mostly because I'm planning to focus on an adventure that mostly takes place in a single massive city, and by making the guards corrupt it opens up the players to tackle more of the challenges in the setting, since the group that should most likely won't.

My Rogue's Guild are actually servants of the Raven Queen, called the Black Feathers. Most of the rank and file do NOT know this, but the Inner Circle actually receives missions from the Shadar-Kai - in fact, if any arcane tricksters have familiars, they are the spirits of Shadar-Kai that have been sent to serve them.

The headquarters of the Black Feathers is a tavern called the Unkind Raven (a group of ravens is called an "unkindness of Ravens"). There's a temple to the Raven Queen in some catacombs under the tavern.

Most of the cities in my homebrew world were once Elven but were taken over by other races after a long-ago cataclysm. The names of places were originally in Elvish but end up evolving - for example, the city that is home to the Black Feathers HQ is now called "Shadebourne" but it was originally Shadar-bornianan, which was the city where the Shadar-Kai ritual that resulted in the Raven Queen.

It was a year+ into the campaign before the players found this stuff out - my rogue player was inducted into the Inner Circle after she already had her familiar - it was revealed then that her familiar was a shadar-kai spirit. Later, there was a mystical even where the rogue got a glimpse of her familiar's true form, so it's been fun trickling this stuff out.

The big bad in my world is a lich (the Plaguebringer) that was the leader of the evil faction of the shadar-kai (he became a lich after siphoning off the power from the ritual). The Black Feathers were created partly to oppose him and take revenge on him, while also serving the Raven Queen's (sometimes very mysterious) directives.