There are two main types of proposals–solicited and unsolicited. In the case of a solicited proposal, a company may release a Request for Proposal (RFP). The RFP will outline the requirements for a specific project and invite vendors to bid, via a formal proposal, to take on the project. On the otherhand, an unsolicited proposal is not a response to a RFP. It is a written submission of details about a particular solution to a company with the purpose of winning a contract.