Request for bid -- During the time of bidding  the vendor will be selected solely based on the price quoted for the work mentioned in the procurement statement of work  for example if there is a tender for constructing bridge the seller who quotes the lowest price will be selected. Request for Bid is a Fixed type Contract

Request for proposal -- Here the buyer is  requesting for price and detailed proposal

Here is some info, i found on net:

http://purchasing.ucsc.edu/howto/quotes&bids.html