IBM 5150 Manual To Operations page 94

Tracks, Bytes, and Sectors
Information is written on the diskette along concentric
circles called tracks. The read/write head of the diskette
drive moves back and forth from one track to another
as the diskette spins over it. This lets the head find
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certain data to read or find a place to write some new
information.
There are 40 tracks on a diskette, numbered from 0 to 39.
DOS reserves portions of track 0 of each diskette. The
rest of the diskette is available for your information and
for a copy of the DOS system, if you want.
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You'll also hear the words byte (pronounced like "bite")
and sector used in talking about diskettes.
Space on a diskette (and in the computer's memory, too)
is measured in bytes. One byte can hold one character;
thus, the 5-1/4 inch diskettes can hold about 160,000
bytes-, or 160,000 characters.
Each track is divided into eight sectors that are 512
bytes long. One or more sector's worth of information
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can be sent back and forth between the computer and
a diskette at one time.
DOS 3-20