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Here's a mnemonic illustration that distinguishes between the two main types of camel:

To be exact, the two-humped Bactrian Camel inhabits the colder regions of central Asia (where it also exists in the wild), while the Dromedary is just the lightest and swiftest of several breeds of the one-humped Arabian Camel (its name deriving from the Greek "dromas", or running, to distinguish it from for example the Pack-Camel variety). Unlike the Bactrian, the Arabian Camel is extinct in the wild, being entirely domesticated long ago.

The two types should therefore be thought of as:

"A.B.C. (not D.)".
ie. Arabian/Bactrian Camels (not just Dromedary)

Taxonomically speaking, camels are related to llamas, vicunas and alpacas. All are mammals in a separate sub-order belonging to the order of Artiodactyls (even-toed or cloven-hoofed animals), being two-toed.