What is WebFinger?
A way to attach information to an email address, or other online resource.
From the official spec:
WebFinger is used to discover information about people or other entities on the Internet that are identified by a URI using standard Hypertext Transfer Protocol (HTTP) methods over a secure transport. A WebFinger resource returns a JavaScript Object Notation (JSON) object describing the entity that is queried. The JSON object is referred to as the JSON Resource Descriptor (JRD).
For a person, the type of information that might be discoverable via WebFinger includes a personal profile address, identity service, telephone number, or preferred avatar. For other entities on the Internet, a WebFinger resource might return JRDs containing link relations that enable a client to discover, for example, that a printer can print in color on A4 paper, the physical location of a server, or other static information.