Villagers-
You can trade with the villagers and harvest food from their farms.
Creepers-
Creepers will explode if you get to close to them, it's best to use a bow. They might give you gunpowder when killed.
Zombies-
Can attack villagers, making them into Zombie Villagers. Zombies burn in light, the sun, regular torches and glowstone will not burn them. They can spawn at night or in caves.
Skeletons-
Shoot with a bow and arrow, usually surprising the player. Spawn at night or in caves, like Zombies, burn when the sun comes up. Both Zombie and Skeleton can avoid burning by being in water, if it rains they will not burn.
Spiders-
Can climb taller structures, but not jungle trees. They can attack up close, and can jump attack, jump at you.
Cave Spider- Can be found in caves, and can poison you while you battle it. Mostly spawn in dungeons.
Jockey-
The skeleton can kill with his bow and arrows, and the spider can climb and attack up close. They don't spawn often, and are very rare.