In an effort for world peace the elders decided that it would be best if they divided everyone into groups. That way everyone would be on equal footing. Everyone was gathered and tested. It was found that there were five major groups:

Abnegation (the selfless) - doctors, nurses, teachers, ect
Amity (the peaceful) - councelors, gardeners, writers, ect
Candor (the honest) - musicians, artists, chefs, ect
Dauntless (the brave) - cops, firemen, EMTs, ect
Erudite (the intellectual) - scientists, lawyers, engineers, ect
Factionless - garbage collectors, maids, store clerks,  ect

But despite their best efforts not everyone fit neatly into these categories. Some people fit into more then one group. At the 16 each everyone was tested and put into the group they were most suited for. Those that fit more then one were ordered to pick a door, the door would determine theor future. You are a 16 year old who is about to be tested.