A C&R license enables you to send/receive Curios and Relics in interstate commerce just like a firearms dealer. Unlike a dealer, you can buy anywhere and sell anywhere, but only Curios and Relics and only when dealing with another C&R licensee. You can't sell to the general public except as an individual. On anything other than C&R's your license is no good.
A Curio and Relic is defined basically as any firearm that is on the BATF Curio and Relic list. This is tricky, since often collectors asked that an item be put on the list so they could sell it. It is not always by type; it could be a Luger, for example, with only certain markings, not all Lugers.
There are automatic weapons on the C&R list, but buyer still has to go through the normal NFA transfer on those so essentially the C&R license doesn't mean much.
A C&R licensee has to keep books, but doesn't have to have a business premises.
The C&R license is sometimes called a collector's license, but this is erroneous; you don't need any Federal license to be a collector.
Because of the limitations and the record keeping requirement, many collectors don't bother. But the C&R license is a great thing for nationally known serious collectors, AKA, high rollers.
Curio and relic lists are available on the web, but I don't have a site right now.
Jim
