A brake rotor looks like one solid disc, but it is really two working regions with very different jobs: the hat and the friction ring. Knowing the difference helps you understand rotor wear, warping, and why some rotors cost more than others.
The hat
The hat is the raised center section - the top-hat-shaped part that bolts against the wheel hub and carries the lug studs through it. It does no braking. Its job is to locate the rotor precisely on the hub and hold the friction ring out at the right position. A speck of rust or dirt trapped between the hat and hub is a common cause of runout - see why to clean rotors before install.
The friction ring
The friction ring is the broad flat outer band the brake pads actually squeeze. On most cars it is vented - two faces separated by internal vanes that pump cooling air through the middle. This is where all the heat and wear happen, and where the rotor's minimum-thickness spec applies - see rotor minimum thickness. When people talk about a rotor being worn or too thin, they mean the friction ring.
Why the distinction matters
Warping and thickness loss happen in the friction ring; mounting and runout problems trace to the hat and hub interface. A rotor that pulses when you brake is usually excess runout, often from debris at the hat - not a "warped" ring - see warped rotor causes. Diagnosing the right region saves you from replacing a good rotor.
Hat design and quality
Better rotors often use a taller or coated hat and precise machining for low runout, and some performance rotors use a separate aluminum hat bolted to the ring. For a standard replacement, a quality one-piece cast rotor with a clean, corrosion-resistant hat is all most vehicles need - browse quality pads and rotor kits and pair them with fresh hardware.