The morning after pill refers to an emergency form of contraception that can be taken by a woman. It is different from the regular birth control pill in that it is taken after a woman has had sexual intercourse with the possibility of not being totally protected against pregnancy. The morning after pill has been the object of controversy, but is used in situations such as when a woman has been raped or where other forms of contraception prove unreliable (e.g., a broken condom).
The working mechanism of the morning after pill is similar to other regular birth control pills in that it contains hormones as well. However, the morning after pill contains much higher doses of hormones, which is the fact that helps it prevent unwanted pregnancy. The name is actually somewhat of a misnomer as it is not only one pill that a woman has to take - and it does not have to be taken the day after the woman has had sexual intercourse. In most cases, doctors will tell you to take the morning after pill the day after sexual intercourse, although it can still work if taken later than that. Another pill - or several more pills - have to be taken hours after the first one in order for the medication to work.
Just like other birth control pills, there are different kinds of morning after pills depending on the formulation and the kind of hormone used. If you find yourself in a situation wherein you need the morning after pill, your best course of action is to see your doctor for advice.