All these terms relate to the use of technology to assist the process of selecting suppliers and purchasing goods. Because this is quite a young industry, many competing terms are in use. For this reason, some definitions may be helpful:
SRM - Supplier Relationship Management . This term is borrowed from the use of technology in Sales, where the term is "Customer Relationship Management". Supplier Relationship Management software is any software that helps an organisation manage its supplier relationships. PostRFP addresses one aspect of SRM - evaluating and selecting suppliers through a tendering process.
e-sourcing - Electronic Sourcing "Sourcing" means to find a source of a good or service. There are two steps in sourcing: identification and selection. Identification involves finding a number of potential suppliers of a good or service. Selection involves selecting the best supplier from these potential suppliers. PostRFP is concerned with Selection, not Identification. Selection can be further broken down into commodity and complex. Commodity purchases can be boiled down to price. Complex purchases are, well, complex, and therefore require many different criteria to be assessed and weighed against one another - an RFx.