An itch is an unpleasant tingly sensation that makes you want to scratch. If you're allergic to poison ivy and you go hiking through a field of it, you'll have a terrible itch all over your legs.
Sometimes people confuse itch and scratch. An itch is the sensation that causes you to scratch, and when itch is a verb, it either means "to feel or sense an itch," or "to have a strong, restless urge," as when you itch to try out your uncle's motorcycle. Somehow, the word itch arose from its Old English root gicce, or gycce, "an itch."