a byte
/ bite /
If a bit is a single light switch, a byte is a panel of eight of them in a row. It is the standard small bundle that computers move around as one unit. When you hear a file is '500 bytes' or memory is '8 gigabytes', the byte is the basic chip we are counting. Almost everything — a letter of text, a small number, one dot of grey in a photo — is stored in one or a few bytes.
A byte is eight bits, so it can hold 2^8 = 256 distinct patterns, conventionally numbered 0 through 255 when read as an unsigned number. People also call a byte an 'octet' to be unambiguous, because on a handful of old machines a 'byte' was a different size. The eight bits have positions, from the least significant bit (worth 1) up to the most significant bit (worth 128); the byte 0100 0001 read as a plain number is 65, and read as text in ASCII it is the letter A.
The byte matters because it is the smallest chunk of memory a typical computer can read or write at one go — you address memory byte by byte, not bit by bit. Sizes are usually given in powers of 1024: 1 KiB (kibibyte) = 1024 bytes, 1 MiB = 1024 KiB, 1 GiB = 1024 MiB. (Disk makers often use powers of 1000 instead, which is why a '1 TB' drive shows up smaller in the operating system.)
The byte 0100 0001 (hex 0x41, decimal 65) means the letter A in ASCII, or the number 65 — same bits, different reading.
One byte = eight bits = 256 possible patterns.
A byte is eight bits on every machine you are likely to meet, but the C standard only guarantees a byte (the size of a char) is AT LEAST eight bits; a few exotic processors used larger bytes.