JWT specifies JSON web tokens. It is defined in RFC 7519: JSON Web Token (JWT)
As indicated in section 3 of the RFC, "JWTs represent a set of claims as a JSON object that is encoded in a jws and/or jwe structure." The claims are name / value pairs where the name is a string and the value can be any JSON value. It is part of the jose framework that also contains JWS and JWE.