For us Americans, think of a crumpet as being like a heavy and moist english muffin.
Didn't help? Okay. It's a pancake with gas. It's a flat cake, maybe 4 inches across, and 3/4 of an inch high, and it's full of air bubbles. It's not as dry as an english muffin. It's not as 'humid' as a pancake. It's not very sweet, but it's usually a little sweeter than an American biscuit.
Not to be confused with a scone.