There are three followers that you meet throughout the Diablo 3 story:
Eirena the Enchantress - An enchantress who casts spells. Crowd control and magic damage. She has a spell that creates illusions.
Kormac the Templar - A templar who holds a spear. He has strong melee attacks and good health. He can tank for you or help you deal extra damage.
Lyndon the Scoundrel - A scoundrel with a crossbow. He is ranged damage with some crow control.
Followers gain XP and change during the story. You get to choose their skills and equip them as they level up, although they are AI controlled. Followers wont be very important in later levels and in multiplayer.
Each follower will have 12 total skills, and players will have to choose which 4 skills from that pool to use, with new skills unlocking at levels 5, 10, 15, and 20. Players will also be able to equip various items to their followers -- there are two weapon slots, rings, amulets, and a unique class item available for each one. As followers level up, they’ll also advance visually, becoming more powerful looking.
When your follower loses too much health, they will go out of commission for a brief period of time. When they are out of commission, you'll lose their benefits temporarily, and then they'll rejoin the battle with you. You won't need to go back to town to have them rejoin you.