For bikes with disc brakes these are the two kinds of brake rotors available.
One thing I gotta know before I start building disc brake wheels is if they are centerlock or 6 bolt hubs. I ask this question a lot and sometimes people don’t know. That’s OK. It’s easy to tell them apart so here’s a photo. Centerlock is a Shimano patented design where the rotor and hub are splined and held together by a single lockring. The six bolt design used by the rest of the world has six torx bolts holding the rotor and hub together.