The morning after pill a form of emergency contraceptive. This is different than "the abortion pill", or RU-486. The morning after pill (which is generally four pills) can be taken after having unprotected sex in order to prevent a pregnancy from occurring. If you’re already pregnant, it does not interrupt the pregnancy. It’s important to get to a pharmacy as soon as possible after having sex and take the medication as directed.
The morning after pill is a great backup plan if you have a condom break, realize you missed a pill, or run into some other situation that leads you to be concerned that you may become pregnant. It can prevent pregnancy 75-95% of the time. It only protects you from the previous encounter and not for the rest of the month. The birth control pill is more like 99% effective, so keep in mind that other forms of birth control are certainly more effective as a "Plan A."
The morning after pill works much like the birth control pill, but in an emergency dose. It contains a high dose of progestin (no estrogen, as in the pill). Being a higher dose than a daily pill would be, women are more likely to experience nausea after taking it.
The morning after pill is available by prescription or over the counter in many areas; it tends to be less expensive with a prescription. It does not protect a woman against any STIs.
Another form of emergency contraceptive is having an IUD inserted. This can be done up to a week after unprotected sex.