In addition to what people have posted above (all very good information), there are some scapulars which denote that the wearer is a Lay/Secular member of a religious order, in this sense they are the equivalent of a religious habit. For example, I am a Lay Dominican, so I wear a plain white scapular. These scapulars are only given to people who are admitted to the order in question, one cannot buy and wear them as an ordinary lay person.
What they are for, (both kinds) is as a religious symbol to remind the wearer (and anyone else who sees it, although they are most often hidden under clothing) of the presence of God in their everyday life and the duty they owe him. In the case of a religious order, this would include specific prayers, actions and solidarity with other members of the order; other scapulars sometimes also have specific prayers attached but they are more usually a reminder to follow Christ and his Church in general. They are a religious symbol similar to someone who wears a crucifix except that in addition they are linked to a particular saint, charism or Marian apparition, so the way in which one choses to live out one’s vocation is guided by that slant.