An octet is a byte of exactly 8 bits. An array of octets, string of octets, or octet stream contains a whole number of bytes, being a multiple of 8 bits.
In ancient history, computers had different sizes for bytes. Different systems had 7-bit bytes, 8-bit bytes, or 9-bit bytes. (The PDP-10 was famous for its 36-bit words, where each word can hold 5 of 7-bit bytes, or 4 of 9-bit bytes.) The word "octet" specifies the 8-bit flavor.
A long time ago, the 8-bit bytes became ubiquitous. (Folks invented the PDP-11, and the 8086, and the 6502, and so on.) These days, a "byte" and an "octet" are exactly the same thing, unless you have an extremely old computer. Many programming languages prefer "byte" over "octet".