Classic is the first, and most basic game mode. It has no goals and you are unable to accumulate resources, thus all blocks are free.
There is no end, nor objective to Classic; it simply gives the player the ability to build freely without limitations. The map is randomly generated, with a range of geographical features and hidden ores. In this mode, said ore has no purpose, whereas they serve a monetary purpose in other Game Modes like Survival.
There are no naturally spawning NPCs in this mode, and recently Notch has removed the use of the 'G' key for placing human mobs; they are apparently to return at a later date for better use. Unlike other modes you cannot die in Creative.