A Cabinet Card is a photograph mounted on a rigid cardstock type backing, roughly 4"x6" in size, most likely an albumen print (but it could be something else), made between the 1870s and 1910s.

Rick A - I did notify the seller and he offered to refund my money. I think I'll just keep the thing, as it wouldn't be worth the time and hassle to return it to London. I only paid $2 for it.