If you have unprotected sex or the condom breaks or something, it stops the sperm cell before it developes into a baby. You have 72 hours after the unprotected sex, after that it is too late. After 24 hours of unprotected sex for the pill to be 100% after 24 hours its not 100% sure that the pill will work. You can only use the pill once a month.
the best chances of it working is using it right away, the sooner the better.beware of the nausea though, you can take a gravol with it. but it's not the most comfortable thing. and i wouldn't abuse it either, it probably takes a toll on your reproductive system.
The day after pill is supposed to prevent you from getting pregnant after having unprotected sex. It won't work if your already pregnant, and it won't abort an already existing baby. You have to use it the morning after having sex, or it pretty much is useless.