A byte is a unit of measurement of the size of information on a computer or other electronic device. A single typed character (for example, 'x' or '8') measures one byte. A single byte generally holds eight bits (bits are, in turn, the smallest unit of storage on a computer, meaning what atoms mean for matter). Bytes are often represented by the letter B.
Historically, bytes have been used to encode text characters. Most of the time, this was done using eight bits per byte, but other numbers of bits were also used. To describe the byte that uses 8 bits, computer scientists talk about octet. EBCDIC is a character encoding used mainly on mainframe computers that uses 8 bits per byte, ASCII is another encoding that uses only seven bits.
When used as a measurement, the byte is basically measuring how many numbers a computer (or electronics device) can hold. This is useful for things like RAM in a computer, or storage devices like USB drives and other types of Flash memory. Sending of data (for a modem or wi-fi) is usually measured in bits, not bytes.
One byte is usually equal to eight bits. Some very early computers had bytes with different numbers of bits. An octet is always eight bits. In modern usage, an octet and a byte are the same.
The symbol for "byte" is "B". Sometimes a lowercase "b" is used, but this use is incorrect because "b" is actually the IEEE symbol for "bit". The
IEC symbol for bit is bit. For example, "MB" means "megabyte" and "Mbit" means "megabit". The difference is important because 1 megabyte (MB) is 1,000,000 bytes, and 1 megabit (Mbit) is 1,000,000 bits or 125,000 bytes. It's easy to confuse the two, but bits are much smaller than bytes, so the symbol "bit" should be used when referring to "bits" and an uppercase "B" when referring to "bytes".