there are 3 formats:
unlimited / modified / limited
unlimited: all pokemon cards are alloud (exept for the famous slowking and sneasel)
modified: all cards in the modified carddex and all cards from pop 1,2,3 are alloud in this format. All big tournaments are held with only cards from these sets, currently it is Hidden legend -on, that means that all cards until hidden legends are alloud to be played. The modified format changes once in a while, the next rotation will be around august-semptember and it will be Emerald on.
limited: for example at prereleases, only cards from 1 set are alloud to be played
i hope that explains enough