Unsolicited proposals are those proposals that are submitted by an investigator in response to a "general call" for proposals that is issued by a funding agency or company in a field or area of study.
The majority of funding agencies issue calls for proposals which have firmly established deadlines and for which the format of the proposals is fairly well defined. Thus, it is vitally important at the outset after you have identified a funding source that you obtain all of the relevant information on the specific grant program and its requirements. Today most funding agencies have searchable websites where they post detailed information concerning their grant programs.