What does the physical structure of a disk describe?

The physical structure of a disk has essentially four features.

1. The number of distinct head positions (cylinders)
2. The number of surfaces on which data can be written (surfaces... and the intersection of a surface and a cylinder is a track.)
3. The number of timing marks detectable when on a single cylinder on a single surface (sectors)
4. The amount of data returned when reading a disk on a single surface and single track between two timing marks. (Sector size a.k.a. block size a.k.a. record size.)

Given sector size, sectors per track, tracks per cylinder, number of cylinders - you have just described the physical layout of a disk. Further, though you can actually choose to read less than the full sector size, the other factors are pretty much fixed by hardware.

A cluster is not a physical structure because you have a choice in cluster size - perhaps not on Windows disks, but certainly on non-Windows disks. A cluster is also called an allocation unit. Clusters are used to reduce the number of bits needed for a complete allocation bitmap. If you represent a cluster with one bit instead of tracking a single disk block with one bit, you need fewer bits to represent all blocks on the disk.

There is one other structural element that SOMETIMES has some significance. If you zero out the geometry registers of the controllers before starting an action, you would seek cylinder 0, track 0, sector 0, which is often called the "home block." This is frequently where special software is stored, or where pointers are found that relate to disk locations of some other significance.

One more hardware item... some disks have a "hardware relocation" section, several tracks set aside as spares in case a track is found to be faulty. Also called an HRT facility, hardware replacement track.