icepick1, to make things simple (and this is assuming that you are debating whether to buy a pci or an agp card...), agp and pci differences have already been pointed out by others. however...
however, from a usage need and purchase point of view, if you are not going to be playing games, just working with business apps or heck, even graphics apps like photoshop, there really is no need to go with agp... if $$ is the issue... since all this involves only 2d and not 3d rendering by the vid. card. vid cards don't require obscene amount of memory for 2d rendering. thus the available vid. card memory (esp on the newer ones) will prove sufficient for the job at hand and the card won't have to use system memory. in this case, there is no point of having an agp (whose main advantage is to increase the bandwidth of the direct channel between the card and system memory.)
bottom line, if you are not going to be playing games, rendering or the like, and if $$ is the issue, go with pci else, agp. one thing i must point out, with the agp becoming the most popular standard for vid card bus format, some pci card prices have started matching those of agp, and sometimes they have even been higher than the agp counterparts. so keep that in mind before u make a purchase for a specific card.