A digital certificate is used in internet security to verify the identity of an entity, such as a website or individual, and to establish secure connections over networks. Issued by a trusted third party known as a Certificate Authority (CA), digital certificates contain information about the key, the identity of the owner, and the CA's digital signature. They play a critical role in protocols like HTTPS, enabling encryption and helping users ensure they are communicating with legitimate sources, thus enhancing trust and security online.