eight bits long

The minor one is that the term `octet' is formally correct but seldom actually used; most people refer to an octet as byte and expect bytes to be eight bits long.

8 bits (a.k.a binary digits)

The basic unit used in the computer world is the byte (a.k.a octet), a byte (or octet) has 8 bits (a.k.a binary digits).

8 bits, which can be used to represent a single letter, number or punctuation mark, or a number between 0 and 255, or anything else that can have up to 256 values

A byte or octet is 8 bits, which can be used to represent a single letter, number or punctuation mark, or a number between 0 and 255, or anything else that can have up to 256 values.