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I've flown several.

Let's talk about airworthiness certificates. Every civilian aircraft registered in the United States must have an Airworthiness Certificate. This document is like the birth certificate of the aircraft, it doesn't expire, it states in broad terms what the aircraft is, who made it and what it's allowed to do. Almost all Cessna 172s are issued a Standard Airworthiness Certificate in the Normal and Utility categories. A Boeing 737 is issued a Standard Airworthiness Certificate in the Transport category.

Each category (In terms of certifying aircraft and not airmen) is a set of definitions and rules the airplane must meet. For example, the rules for Normal, Utility, Aerobatic and Commuter classes are found in FAR part 23. The aircraft has to conform to a type certificate. The manufacturer invents a make/model of airplane, they go to the FAA to get it certified, the FAA issues a type certificate which accurately describes that make/model, and now they can manufacture them and certify them as that type.

It gets BITCHY. The airplane must, at time of manufacture and for eternity thereafter, conform to the type certificate. Which means modifying a type certified airplane is a bit of a bitch. Modifications beyond things like paint color have to be done under a Supplemental Type Certificate (STC). A certified mechanic has to do the modification, per the instructions in the STC, and then that STC has to live with the airplane's paperwork forever. There is an STC for a modified sun visor for the Cessna 172. They don't fuck around.

Experimental is for everything that doesn't go through that asspain of paperwork. Yes, it calls up the image of a daring test pilot trying to tear the wings off an X-42, but...also Cessna had to test fly a Cessna 172 for awhile when they switched from Continental to Lycoming engines. That aircraft was flown under an Experimental-R&D airworthiness certificate.

Homebuilt aircraft, including kit-built, are certified under a Special Airworthiness Certificate in the Experimental-Amateur Built category. To get that, the builder of the aircraft has to meet with a Designated Airworthiness Representative who will look at your documentation of the build process, your receipts for materials, perform a detailed inspection of the aircraft itself, and then issue the certificate with its operating limitations. There will be a test flight phase and an operating phase. This aircraft will be prohibited from commercial use and certain other operations, but you don't have to go through the ass pain of all that certification. They also issue the builder of the aircraft a repairman certificate allowing them to perform their own annuals on THAT airplane.

Then there's warbirds. The military has its whole own thing with aircraft, the FAA has issued no airworthiness certificates to F-22s. Yet. It depends on the warbird here: some WWII aircraft were existing civilian models, like the L-4 Grasshopper was an olive drab Piper J-3, or the C-47 Skytrain was a Douglas DC-3 with an extra big door. When those were sold to civilians after the war they were certified as Standard under Normal and Transport categories. For a few military only designs the Limited category was invented, and for weird one-offs like...if you buy a MiG, or that outfit out in Nevada that has a fleet of civilian F-16s, they're operated under either Restricted or Experimental-Exhibition categories.

The last one I'll blabber about is Experimental - Light Sport Aircraft, which are a weird little case where the manufacturers of Special Light Sport Aircraft can sell almost complete kits. Some of them, you roll the plane out of the box, bolt the wings, tail and propeller on, and call the DAR. It has to be identical to an existing SLSA, but doing it as ELSA means you give up the limited commercial uses it has (SLAs can be used for commercial flight training operations) for pilot-owner maintenance.

end infodump