The ATP Challenger Tour, known until the end of 2008 as the ATP Challenger Series, is a series of international men’s professional tennis tournaments. The Challenger Tour events are the second-highest tier of tennis competition, behind the ATP Tour. The ITF World Tennis Tour tournaments on the entry-level of international professional tennis competition. The ATP Challenger Tour is administered by the Association of Tennis Professionals. Players who succeed on the ATP Challenger Tour earn sufficient ranking points to become eligible for main draw or qualifying draw entry at ATP Tour tournaments. Players on the Challenger Tour are usually young players looking to advance their careers, those who fail to qualify for ATP events, or former ATP players looking to get back into the big tour.
source: wikipedia
