RFx stands for "Request for X", where X can is a variable that can take different values:

RFI — Request for Information

The most common type of request, often a part of a tender procedure. An RFI response may contain a general company or product information, relevant case studies, references, pricing details, and other items giving the client a better understanding of the vendor's fitness for the job.

RFP — Request for Proposal

The most common type of request. The response contains a commercial proposal based on the company's understanding of the client's requirements. The proposal may be supplemented with additional materials, such as case studies or references.

RFQ — Request for Quotation

A stripped-down version of an RFP with the overall cost of the project being the highlight of the document. An RFQ can be an estimation with a list of assumptions affecting the cost of the project and a list of additional services required for the solution.

RFB — Request for Bid (of Invitation for Bid, IFB)

A rare form of a proposal used primarily for tender procedures. The focus is shifted from providing alternatives and coming up with unique solutions to meeting the exact client's requirements and complying with the procedure. Often takes the form of an RFQ.