Indicas give a more relaxing body high, almost narcotic. Like horror business said, you get a very comfortable feeling. You tend to just want to sit and chill and are more inclined to look for or the nearest couch (hence the term "couchlock") than the nearest party.

Sativas give you an up, happy high. Your mind is definitely more affected. You'll be more energetic and more likely to want to do something.

Indicas tend to be a bit darker in color. If you've got some really dark green, or purple buds it is likley to be an indica. Sativas tend to be a bit lighter in color. There are exceptions, however, and a lot depends on how the weed is grown/dried/cured so color alone isn't a perfect indicator.

I like both indica and sativa and usually choose depending on what I am doing. If it's night time and I'm just kicking back at home I like a good relaxing indica. If I'm doing something active or something like yard work, however, a more energetic sativa is the way to go.

Most of the weed grown/sold in the U.S. is a cross breed of both indica and sativa.