I would say there there are basically two kinds of "clone":
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Shield-compatible clones. In addition to duplicating the electronic content of the arduino, a shield-compatible clone will have connectors in the same physical locations so that the "shields" can be attached normally. Not all of these have the same physical size as an official Arduino, but the main connectors will be in the right places. Examples of this sort include the NKC Freeduino and the recently announced "Induino" (http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1267541811) (which is interesting because of its relatively drastic departure from the normal size and shape.)
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clones that are NOT shield-compatible. Usually these have some other physical mounting goal in mind. For example, MDC's "Bare Bones Board" and Adafruit's "BoArduino" are designed to be usable with protoboards, and the lilypad is "wearable"...