Some say there are two types of circular arguments:
a) Vicious in which the conclusion is included in the premise, this type of circularity has little or no explanatory value.
b) Non-vicious circularity where part of the premise is also included in the conclusion, but in this circular argument information is added by the argument. Example 'T is the largest man in the the room'. T is also in the room but it is not viciously circular because it adds to our understanding about T.

God exists because the Bible says so. ( This is NOT my favourite circular argument. In fact it irritates the crap out of me but it was the only one I could think of)