I guess for me it is just being interested in cars. Even if you don't drive cars regularly (too young, too poor, living in Manhattan or some other place where driving daily isn't very practical) if you are interested enough to spend a lot of time in car forums, reading Car & Driver or Motor Trend cover-to-cover, easily spending hours on AutoBlog or Jalopnik, or trolling your local Craigslist auto forsale by-owner looking for deals or rarities....
I consider myself an auto enthusiast even though:
-I drive a 2000 Saturn 4-door with 100hp that I paid $3k for
-I have only ever driven a couple of relatively high-performance vehicles in my life (late 90's Z-28 and Formula Firebird a couple of times, plus a C5 once)
-I haven't ever been to an auto racing event of any kind, and don't keep up with them at all
-The most mechanical work that I have ever done personally is to replace the cylinder head on a 4cyl Saturn, and the intake manifold on a Chevy V8
I love cars, I love reading about cars, I love talking about cars, I love knowing how they work. I have several other similar "hobbies" (guns, cameras, computers) but I always end up returning to cars.