Someone post a screenie of the zombies when they get the chance.
Some of these are a little dark I guess, but you get the basic idea. All enemies spawn in the dark, not in well lit areas. So unless you're in a cave or underground somewhere, monsters spawn only at night.
ZombiesLocation: main world
Attack Type: melee
Sunlight Reaction: sunlight sets them on fire, killing them
SkeletonsLocation: main world
Attack Type: they shoot at you with a bow and arrows
Sunlight Reaction: Sunlight sets them on fire, killing them
SpidersLocation: main world
Attack Type: melee
Sunlight Reaction: None. They can still attack you freely in the sunlight.
CreepersLocation: main world
Attack Type: They get close to you, then explode, leaving a crater.
Sunlight Reaction: None. They can still attack you freely in the sunlight.
Notes: The explosion damage is enough to kill you in one shot if they're right up against you. They hiss (like the sound of a fuse burning) just before they blow up. To kill them, you either shoot them with arrows, or use a hit and run tactic; if you get in close enough to hit them with a sword, then back up as they get knocked back from the blow, they will stop hissing and not explode.
Zombie PigmenLocation: The Nether
Attack Type: melee
Sunlight Reaction: Unknown. I don't think it's possible for them to spawn in the main world, and they can't travel through portals.
Notes: They are passive unless attacked. When they attack, they attack in groups. So if you attack one, all the ones around him will attack you.
GhastsLocation: The Nether
Attack Type: They shoot fireballs at you, that ignite any surface they hit, and destroy most block types within a small radius of the impact.
Sunlight Reaction: Unknown. I thought I'd read that a ghast can randomly appear near your portal in the main world, as if he came through it, but I've never seen it happen.
Notes: It may not look like it from the picture, but they are HUGE. they're about 8-10 blocks in size, square, plus the tentacles. They fly, and attack from a pretty big distance. Also, most of the blocks in The Nether are what I call brimstone. Brimstone will catch fire and burn forever, so be careful.
Out of all of these, the Creepers and Ghasts are the most deadly. Creepers make no sound until the hiss, so they often manage to sneak up on you. The ghasts just relentlessly shoot fireballs at you from long distances, setting the environment on fire, and destroying blocks around you.