@robbie21 - The opposite of striated muscle is smooth muscle. Contraction of smooth muscles is involuntary; you don't control it, which is probably why you don't think of smooth muscle when you think of "a muscle."
Smooth muscle is found in lots of places in your body, including your blood vessels and your digestive tract. These internal systems use contraction to move things along, digest your food, etc., but you don't have any control over it and usually don't feel it. One exception: The uterus is a giant smooth muscle (that's why OBs talk about "contractions") and that one, people feel when it contracts!