Followers match the level that you are when you meet them, but they don't level with you.Just remember to take the torches away from them or have them wait somewhere when you are trying to sneak, as the light will make sneaking a lot less effective. Give your follower a torch and they will use it when it's dark.
So warrior followers get heavy armor, 2 handed swords, mages get staves, archers get bows, and so on.
They will equip the best items from what you give them, but make sure it suits their class. However, some quests allow additional followers to join you temporarily. Here is the list of followers you can find in Skyrim. Once you complete that task, they will join you for good. Most followers have a task you must accomplish before they join you. They fight with you, carry your things, and stay loyal to you no matter what.
In Skyrim, followers (or companions) are more usable than ever before.
