COBOL programmers are used to working with memory physically organized into groups of eight bits, and calling these groups bytes. However, in communications, where we are talking about a continuous stream of bits on the wire, each group of eight bits is called an octet.

The ASN.1 encoding rules define four types of octet: identifier octet, length octet, contents octet, and end-of-content octet. There's no need to go into detail here about exactly how these are used (it's different for different encoding rules anyway); it suffices to say that the contents octets contain the actual data, while the others are needed for the syntax of the encoded bit-stream.