Difference between RPC and Doc. type in WSDL(need help)

Hi,
    We are going to develope a tool to convert Web services in to WSDL.
For that I want to know the exact differences between RPC type and
Document type in a WSDL file(Specially in the complextype area in the
WSDL file). I read so many documents but I didn't find any exact
answer for that. most of the documents didn't talk about complextype
area. So, can anyone give me solution for this.
    Please help. Thanks.