The National Tennis Rating Program (NTRP) classification system is a rating from 1.0 to 7.0 used by the United States Tennis Association (USTA) for their competitive leagues and tournaments. The Universal Tennis Rating (UTR) system is a rating from 1.0 to 16.0 that is used worldwide and is based on your results in UTR-sponsored matches and tournaments.
Both rating systems are best determined by match play, but it's hard to determine either rating without playing in officially sponsored matches, especially if you just want to play tennis recreationally!
TopSpin provides a self-rating system based on Tennis Canada's self-rating guide but distilled into an easy-to-use 20 questions quiz.