This is an excellent treatment of the topic by L. Detweiler titled: Identity, Privacy and Anonymity on the Internet.  An exerpt:

Simply stated, anonymity is the absence of identity. However, there are several variations of anonymity. A person may wish to be consistently identified by a certain pseudonym or 'Handle' and establish a reputation under it in some area, providing pseudo-anonymity. A person may wish to be completely untraceable for a single one-way message (a sort of 'Hit-and-run'). Or, a person may wish to be openly anonymous but carry on a conversation with others (with either known or anonymous identities) via an 'Anonymous return address'. A user may wish to appear as a 'Regular user' but actually be untraceable. Sometimes a user wishes to hide who he is sending mail to (in addition to the message itself). The anonymous item itself may be directed at individuals or groups. A user may wish to access some service and hide all signs of the association.

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