A byte is a sequence of eight bits processed as a single unit of information. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit of memory in many computer architectures.
Most hard drive manufacturers define maximum storage using the official SI prefixes (e.g., kilo, mega, giga, tera, etc.), but the operating system defines the size of the hard drive using the binary powers of two definition (technically incorrect!).