In a nutshell the seller (grantor) on a quitlcaim deed is giving up any claim he or she has to the property in question. Usually on a quitclaim deed the seller has a questionable claim or the buyer is 100% certain the seller has a claim (usually because the seller is an heir to the property and the buyer is as well). On a warantee deed, the seller is guaranteeing that he has clear title.
We have posted two articles on our site regarding quitclaim deeds and their use in genealogical research. Both article one and article two can be linked to on our site.