Pay-to-Play (P2P) games require you to pay a fee to pay. It's usually a monthly fee like $12/month.

Buy-to-Play (B2P) games require you to buy the game to play. I guess this can be said for almost every game you buy at the store, but I like to group them with games like Guild Wars and Diablo 2. You buy the game which has a CD-key which can be used to make an account without a monthly fee.

When professional websites such as our very own MMOsite marks something as B2P it always means that you only have to buy it to play it, but some games will require you to pay a monthly fee for the online features but allow you to freely player the single player modes for life (For example, Phantasy Star Universe). If it doesn't specify that it is just buy to play and nothing more, then you should ask around the game's forums or a general gaming forum, also Gamestop employees are usually very knowledgable about most games (However some can also pretty stupid sometimes :/  ).