An encoding descriptor is something which can be used with binary input or output streams to encode or decode bytes stored in a certain representation. It must conform to the Encoding protocol. Encodings which you can use are defined in the following vocabularies:
Binary encoding
UTF-8 encoding

UTF-16 encoding

UTF-32 encoding

Strict encodings
Legacy encodings:
Legacy 8-bit encodings

ASCII encoding

See also
An introduction to encodings