FFL is a commonly used acronym for Federal Firearms License. There are several FFL variants that encompass most facets of the firearm and ammunition trade in the United States: Buying and selling new and used firearms, gunsmithing, collecting antiques, the manufacture of firearms, ammunition, and sale and manufacture of things that go boom and make craters in the ground. Everyone from the tiny gun shop across town, the online retailer (like us!), to the big-box sporting-goods retailer has some form of FFL if they are engaged in the business of firearms.
This system was born of the Gun Control Act of 1968. It laid the foundation for ownership and purchasing requirements and created the federal firearms licensee system that effectively regulates all interstate commerce of firearms. All firearm purchases excepting some examples between private individuals, must be completed through an FFL. Additionally, all handgun purchases must be completed in the state of the acquirer's residence.