A sweeper is usually a pokemon with Frail defenses and usually high speed made for attacking the opponent and fainting as many pokemon as possible. Some examples are typhlosion, alakazam, Weavile, and Rampardos. These guys can't really take hits, so they do as much damage as they can before going out.
A tank is a pokemon usually with lower speed but good defenses. These guys can take hits as well as dish them out. Some examples are Rhydon, Slowking, scizor, and Magnezone.
With these terms though, remember that a pokemon isn't limited to just one. A pokemon can usually perform several forms and function however you deem fit for its abilities.