RFC 6750: OAuth 2.0 Bearer Token Usage

tools.ietf.org/html/rfc6750

Bearer Tokens are the predominant type of access token used with OAuth 2.0.

A Bearer Token is an opaque string, not intended to have any meaning to clients using it. Some servers will issue tokens that are a short string of hexadecimal characters, while others may use structured tokens such as JSON Web Tokens.

More resources