php .vs cgi app..

hi.. a really general/basic question... what is the difference between a "cgi" app and a php app.. does it really come down to where the app is being run from.. i mean within apache, if i specify that php/perl/etc... app resides at a given location, and that files with a certain extension are to be handled by the perl/php/etc app, then i can handle the processing of the file by the interpreter anywhere within my site that i grant access to. is there anything/any reason to have a "cgi-bin" portion of my app, other than good design practice... clarification would be useful!!! thanks.. -bruce