The fuzzer basically allows you to throw a combination of values at a server. It is still a little rough, but functionally, it works pretty well. The idea is that you configure the request method, the basic URL (without any parameters), the request version, any headers (e.g. a Host: header if you are using HTTP/1.1, etc. Don't configure cookies here!), and a list of Parameters.
A Parameter is defined by its location (Path, Fragment, Query, Cookie, Body), its name (ignored for Path), type == String, Default value, Fuzz Priority, and a fuzz source.
The default value is the value that will be submitted if no fuzz source is defined.
The fuzz priority defines how the various fuzz sources are combined: if all the priorities are the same value, the number of queries submitted will be the number of items in the shortest fuzz source. If they are different values, the number of queries generated will be the product of the number of items in each level.
An example would be if you had a list of usernames and passwords. If both username and password parameters had the same priority, usernames and passwords would be consumed in lock-step. If they had different priorities, EVERY password would be tried for each username.
Parameters are processed in order, first path, then fragment, then query, then cookie, then body. The only content type supported for POST Requests is application/x-www-urlencoded, it does not support multi-part (although provision is made for parameters of different types, e.g. File), or XML documents, etc. Multi-part and XML will be the next evolution.