- Root Certificate Authorities (CAs): These are the top-level entities in the trust hierarchy, responsible for issuing and managing digital certificates. Root CAs are highly trusted and their certificates are pre-installed in browsers and operating systems. - Intermediate Certificate Authorities: These CAs are certified by Root CAs. They can issue SSL/TLS certificates to end entities, such as businesses or individuals. Intermediate CAs help in distributing the trust model, making it more scalable and secure. - Issuing Certificates: CAs validate the identity of entities before issuing certificates. The validation process ranges from basic domain validation to more rigorous extended validation for higher security. - Certificate Lifespan and Renewal: SSL/TLS certificates have a set validity period. Entities must renew their certificates periodically to maintain secure communications. - Revocation and Transparency: Certificates may be revoked for various reasons, like compromise or loss of private keys. Certificate Transparency (CT) logs are maintained to ensure public visibility and tracking of all issued certificates. - Browser and OS Trust: For a certificate to be trusted by default, the issuing CA must be included in the trusted root store of browsers and operating systems.