Constant Angular Velocity (usually known as CAV) is the way (most) floppy disks are written, and hard disks. See also Constant Linear Velocity for the alternative system.

Essentially, a disk which is being read or written in a CAV system spins at a constant rotational speed - typically about 300 RPM for floppy disks, and anything from 3600 RPM to 15000 RPM for hard disks. The advantage of this system is that it's very simple to make as the motor spins at a constant speed!

However, it has a major disadvantage. Let's say it takes the head 1ms to write a sector of data (a wildly inflated figure, but it makes the calculations easier). And let's assume our disk is spinning at 6000 RPM. This means that it spins 100 times / second, or alternatively covers 1 revolution in 1/100 of a second or 10ms. So, if it takes 1ms to write one sector, it can write 10 sectors per track on the disk. So far, so good.

But not all the tracks are the same length. In a 3.5" hard disk, the innermost sector is (let's say) 0.5" from the centre of the spindle. This means it's about 3.14" long (circumference of a circle = pi x the diameter). So each sector in the above example, each sector is about 0.314" long, and the head has to be able to write 1 sector's worth of data in 0.314". On the other hand, the outermost track may be 3" from the spindle. This track is about 7" long - twice as long. But the drive can still only write 10 sectors in this space, therefore wasting half of the possible capacity.

This is why CAV is an inefficient way of using disks, and why some systems (most notable Macintosh floppy disks, and CD-ROM discs, use CLV instead.

Some modern hard disks use a half way system. The disk rotates at a constant speed, but the disk is split into 2 or more concentric zones. In the innermost zone, the system may spend 1 ms writing a sector. In the next zone, it may take 0.5 ms to write a sector, therefore using the same amount of disk space as it takes using 1 ms in the small tracks. And so on. This system is called "Zoning", and is used almost universally on modern disks. It's also why standard "Cylinder/Head/Track" size specifications are meaningless for modern disks - different tracks have different numbers of sectors on them. But the drive does all the clever work in the background.