A Request for Information (RFI) is used when you think you know what you want but need more information from the vendors. It will typically be followed by an RFQ or RFP.
A Request for Quote (RFQ) is commonly used when you know what you want but need information on how vendors would meet your requirements and/or how much it will cost.
A Request for Proposal (RFP) is used when you know you have a problem but don’t know how you want to solve it. This is the most formal of the “Request for” processes and has strict procurement rules for content, timeline and vendor responses. The appendices in the Guide to Successful Software Acquisition provide templates for this approach.