A byte is a unit of measurement of the size of computer, or other electronic data (information). A byte is two hexadecimal digits. When used as a measurement, it 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.
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".
Names for larger units [change]
For large data, byte is often used with a binary prefix:
Byte Chart [change]
According to the International Electrotechnical Commission (IEC), who sets many computer standards, these charts show how bytes should be referred to.
People who refer to 1 kilobyte as 1,024 bytes, for example, are technically incorrect; 1,024 bytes should be referred to as 1 kibibyte, according to the IEC.  However, using 1024 for kilo and 1048576 for mega, etc. was widely practiced before the IEC standards were set in 1998. There is some confusion and mixing of terms in the marketplace.
"kilo-" = 1,000 [change]
|Byte (B)||1||1 byte|
|Kilobyte (kB)||1,000||1 thousand bytes|
|Megabyte (MB)||1,000,000||1 million bytes|
|Gigabyte (GB)||1,000,000,000||1 billion bytes|
|Terabyte (TB)||1,000,000,000,000||1 trillion bytes|
|Petabyte (PB)||1,000,000,000,000,000||1 quadrillion bytes|
|Exabyte (EB)||1,000,000,000,000,000,000||1 quintillion bytes|
|Zettabyte (ZB)||1,000,000,000,000,000,000,000||1 sextillion bytes|
|Yottabyte (YB)||1,000,000,000,000,000,000,000,000||1 septillion bytes|
"kibi-" = 1,024 [change]
Since computers are digital devices that are based on the binary numeral system rather than the commonly-used decimal numeral system or binary coded decimal system, there are many situations where the standard metric system does not work well, particularly with memory sizes for a computer or storage device. If a memory or storage device uses a binary number for addresses, the number of different positions to be accessed (the size of the memory) can be expressed as a power of 2, rather than a power of 10.
|Byte (B)||1||20 bytes|
|Kibibyte (KiB)||1,024||210 bytes|
|Mebibyte (MiB)||1,048,576||220 bytes|
|Gibibyte (GiB)||1,073,741,824||230 bytes|
|Tebibyte (TiB)||1,099,511,627,776||240 bytes|
|Pebibyte (PiB)||1,125,899,906,842,624||250 bytes|
|Exbibyte (EiB)||1,152,921,504,606,846,976||260 bytes|
|Zebibyte (ZiB)||1,180,591,620,717,411,303,424||270 bytes|
|Yobibyte (YiB)||1,208,925,819,614,629,174,706,176||280 bytes|