This is from the FAQ at A Smokers Guide :

10. "What is the difference between Indica & Sativa?"
Indica and Sativa are both cannabis strains. Indicas are notably squatter with more crystals while Sativa is more elongated with its crystals kept in the core. Indicas are much more heavier than sativas. The high is more of a body stoned effect versus a more cerebral high from sativa. Many feel that Indica is going to make them sleepier, while Sativa gets you UP"

A lot of strains you might buy will be a cross between the two.