As you play through the game and help people out, many of them will wish to help YOU out. You can find all sorts of followers and companions and have them follow you around Skyrim and fight your enemies with you. This list is here to help you figure out who you can get to follow you.
Note that followers all have their own skill-set. Some are magicians, some are archers, some are warriors and so on and so forth. They also have their own levels of health, stamina, and magic. You can't manually affect any of this, but your followers ARE affected by when you meet them, as they can be stronger if you meet them at a higher level compared to a lower level.
Followers are a HUGE asset if you're OK with having someone else around. For example, they can carry stuff for you, train you (if they are a trainer), and will even use better equipment if you give it to them. They will also do whatever else you ask of them, including stealing, attacking NPC's, and lock picking (only CERTAIN followers will do these things of course). However, not EVERYTHING is peachy. Followers can get in the way, have trouble following you around, and mess up your own sneaking and stealing efforts if you're trying to be stealthy, so think long and hard about whether or not you want a companion.
Companions are sorted by Male and Female down below, mainly because I really don't have any better way to sort them. There's also "other" types of companions you can have below the males (think dogs). You can only have one companion legitimately at a time, but you CAN have a companion AND an animal friend if you wish. Horses also act much like companions, but they don't follow you around in dungeons and whatnot, so I didn't include them here. I also don't include temporary companions that you may find in quests and that only follow you around for a limited time. So with all of that in mind, onto the followers list!