They are closely related.
The easy way to remember which is which is the Pembroke's tail is broke.Stupid, but it works to keep the separate. In fact the tail is not broken, the breed has a VERY short tail.
Sometime it helps to refer to the breed standard when you have 2 you are trying to tell apart.
Pembroke:
http://www.akc.org/breeds/pembroke_w...orgi/index.cfm
Cardigan:
http://www.akc.org/breeds/cardigan_w...orgi/index.cfm
On each page, the links to the standard, to colors and markings, etc. appear on the far left
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