If a Ghost type uses Curse the target is the foe and the foe will lose 25% (1/4) of its maximum HP at the end of every turn until it faints or leaves battle. This effect can be Baton Passed to other teammates, so be careful... The Ghost type that used Curse on a foe will lose 50% (1/2) of its maximum HP when the move is used.
If any other Pokémon uses Curses the taget is the user and the effect is given to the stats of the user. The user will not lose any HP. Instead the user's Attack and Defense stats are boosted 1 stage (x1.5) and the Speed stat is reduced 1 stage (/.5). Cursing for stat boosts was a more popular strategy in the third generation metagame than it is now, though those Pokémon, like CurseLax (a Snorlax using Curse), still see some play.