A JSON Web Token (JWT) is an open standard for securely transmitting information between parties as a JSON object. It defines a compact and self-contained way for creating data with optional signature and/or optional encryption whose payload holds JSON that asserts some number of claims. JWTs are designed to be compact, URL-safe, and usable especially in a web-browser single-sign-on (SSO) context. They are commonly used to pass identity of authenticated users between an identity provider and a service provider, or any other type of claims as required by business processes.

A JWT typically consists of three parts: a header, a payload, and a signature. The header typically consists of two parts: the type of the token, which is JWT, and the signing algorithm being used, such as HMAC SHA256 or RSA. The payload contains the claims or the JSON object. The signature is a string that is generated via a cryptographic algorithm that can be used to verify the integrity of the JSON payload.

JWTs are used in various scenarios, including authorization and single sign-on (SSO). Once a user is logged in, each subsequent request will include the JWT, allowing the user to access routes, services, and resources that are permitted with that token. JWTs are more compact than other token types, such as Simple Web Tokens (SWTs) and Security Assertion Markup Language Tokens (SAML). They are also efficient and stateless, which means they dont require a database lookup to verify.

In summary, a JWT is a secure and compact way of transmitting information between parties as a JSON object. It consists of a header, a payload, and a signature, and is commonly used for authorization and single sign-on (SSO) purposes.