What is the initial save of this spell

@Skudmora1: It would depend on the caster. The spell Slow  can be cast by both Wizards and Sorcerers. According to the Basic Rules, (which I heavily recommend reading before building your character,) these two classes use different Abilities when casting spells which affects the DC, but not what Ability (in this case Wisdom) is used when rolling for the Save.

The Spellcasting Ability of Wizards is Intelligence whereas Sorcerers use Charisma. Unless specified otherwise, the Save for your spells is the following:

Spell Save DC = 8 + your proficiency bonus + your [Spellcasting Ability] modifier

Be sure read the rules and understand them before getting into character creation. If you need answers to any questions, I recommend turning to the RPG Reddit or Stackexchange for more immediate answers.

Would a creature falling that has Slow cast onto them be falling at regular speed or would it's fall be slow as well? 

Time is slowed for the creature, not the air beneath it. Their ability to react to the fall would be in slow motion, but the fall itself would be in standard real time. The telling detail is -2 dex save, that’s literally representing being slower to react to the real time flow around a character or creature.

The creature would fall at the standard 500ft per round. Slow will potentially cause that target to not be able to cast a life saving spell such as as Feather Fall, Fly or Dimension Door.

The spell seems to contradict itself. The first paragraph: Each target must succeed on a Wisdom saving throw or be affected by this spell for the duration.

But at the end it says 'A creature affected by this spell makes another Wisdom saving throw at the end of each of its turns. On a successful save, the effect ends for it.'

Can anyone clarify which one is correct?

Both. They make the first wisdom save and then get to do it over and over until they succeed

One of the most underrated spells in the game! This can be a game changer, especially if you coordinate with another player who can compromise their ability to make the WIS save, such as by casting Bane. Can also ruin the day for enemy spellcasters.

Do spells or terrain that affect speed compound.  For instance, Plant Growth cut land speed by 1/4.  Spiritual Guardians cuts it by 1/2.  Slow cuts it by 1/2 as well.  SO would the speed of the creature be 1/16th norm if affected by all three?

Difficult terrain doesn't "stack;" either there is difficult terrain or not. However, some terrains specify that moving through it requires either 2 ft of movement for every 1ft moved, or 4ft of movement for every 1 ft moved.

If there are multiple sources of difficult terrain being stacked in one location, only the highest speed reduction would apply; not collectively (additive).

Finally, since Slow doesn't create difficult terrain, but instead directly halves the opponent's movement speed, it would indeed be compounded if difficult terrain were to be traversed by said character while under the Slow effect.

FOR EXAMPLE: 
A druid casts Plant Growth around a baddie, then a sorcerer casts Slow on said baddie. If that baddie originally had a movement speed of 40 ft, it would be immediately reduced to 20 ft without even attempting to move. If they then try to move, it would cost them all 20 ft of movement to move just one 5ft square. If a nearby Cleric then casts Spirit Guardians on itself, the baddie would take the damage, but it would still have 20 ft of movement speed, and it would still require the baddie using 20 ft of movement just to move 1 ft square.

Quote from littledijon >>

Do spells or terrain that affect speed compound.  For instance, Plant Growth cut land speed by 1/4.  Spiritual Guardians cuts it by 1/2.  Slow cuts it by 1/2 as well.  SO would the speed of the creature be 1/16th norm if affected by all three?

The not all effects of those spells stack, but you could still slow someone down a lot.

Cut off 3/4, or even 7/8, of the distance they’d usually move each turn.

mini Za warudo

Shouldn't this spell be also available for bards?
Ref: Tasha's Cauldron of Everything, page 27, Additional Bard Spells.

Agree with lordashendale

Note that those are *optional* rules. They are to be applied at the Dm's discretion, and as such aren't included in the base spell description.

It can't be both, look at the wording...

Is it - Each target must succeed on a Wisdom saving throw or be affected by this spell for the duration.

Or is it - A creature affected by this spell makes another Wisdom saving throw at the end of each of its turns. On a successful save, the effect ends for it.

Being affected for the duration doesn't mean being incapable of breaking free of its effect. 

This spell probably isn't useable against dragons, right? I'd love to be able to limit their action economy and AC, but even if they don't outright succeed on the wisdom save they can just choose to succeed anyway? 

There’s nothing preventing a caster from using this spell on a dragon. They’re creatures! The issue with doing so effectively is twofold. 

First, like you pointed out, adult and ancient dragons of all colors have legendary resistances, meaning they can just “no ❤️“ this spell. 

Second, dragons of any age tend to have pretty decent Wisdom saves for their challenge ratings. 

Dragons (and any creature with multiple attacks and high movement speed) are good candidates for this spell, but they’re harder than others might be to have it take hold. 

Quote from Varis_Galanodel1855 >>

This spell probably isn't useable against dragons, right? I'd love to be able to limit their action economy and AC, but even if they don't outright succeed on the wisdom save they can just choose to succeed anyway? 

 Not if you burn their legendary resis. And if youre luck and have a good DC (maybe casting Bane would help), it could fail. Also, Ledgendary Resistance is only for Adult and Ancient dragons.