Emergency contraception is used after sex if a woman is worried that she will fall pregnant. There are two main options for this - either oral emergency contraception pills or the copper IUCD (intrauterine contraceptive device). The pills contain either a progesterone alone (e.g. Plan-B), or else the progesterone plus oestrogen. Pills are much more commonly used, and the most effective contain a progesterone alone.
So, is emergency contraception safe? Bascially, yes.
However, it is meant only in an emergency (e.g. when the condom breaks, etc.). One shouldn't need to take them with any regularity - it is far better to use conventional contraceptive methods (e.g. oral contraceptive pill, IUCD, Mirena, etc.).
The reasons for this are twofold: Firstly, the failure rate with emergency contraception is higher than most conventional contraceptive methods. Thus, if you are looking to prevent pregancy over the long run (as opposed to when other contraceptives fail), then emergency contraception is actually worse than most other long-term contraceptive options. There is a place for it or course, but that should only be when conventional contraceptives fail (e.g. broken condom).
Secondly, the dose of progesterone (and often oestrogen) in the pills is actually higher than the level used in conventional contraceptives (e.g. combined oral contraceptive pill, Mirena). This means increased risk of side-effects (although they are usually fairly mild).
Hope that all helps :)