from my experience its 3 things, wether it is single beam or both beams.
What type of bulb? my OEM 99 integra headlights requried the 9006 bulbs. I bought a pair of aftermarket halo projector headlights, and even though the description of the headlights on the webpage had a ton of "OEM, OEM, OEM," it changed the type of bulb. The only thing that was OEM was where they bolted on, and the wiring harness for the "plugging in." If you make a mistake, you can probably send back the bulbs for the correct bulbs you need, if you dont use the bulbs.
The next thing is the temperature of the bulb, or color. The seller will probably tell you which temp. is which color, so you just choose what you want.
And the last is the ballast, slim or regular. Slim are smaller, obviously, so it is easier to mount.
Other than that, its simple. Just "plug and play."