International Master (IM) is the second highest hierarchical title awarded in chess (see Grandmaster). Requirements include three International Master norms (performances to International Master standard in FIDE rated tournaments) within a 5-year period, one of which must be in an all-play-all event, and the raising of one's ELO rating above 2400.
Like the Grandmaster (GM) title, the International Master title, once awarded, is retained for life. It is not as prestigious, obviously, to be an IM as it is to be a GM, however it is a significant milestone in a chess player's career, as it marks the point after which he or she may expect to be invited to play in international tournaments for free, with accomodation paid. Professional chess players are, famously, perpetually penniless, and appreciate these small kindnesses.
Other titles include:
FM (Fide Master)
WGM (Women's Grandmaster)
WIM (Women's International Master)