Acting like a driver's license or a passport, a certificate provides a generally recognized proof of a person's identity. CAs can be either independent third parties or organizations running their own certificate-issuing server software. A private certificate may be used by a private entity for use in their VPN while the Internet, which is a public network, is generally served by public CAs. Each certificate acts as a digital identification card. The certificate issued by the CA binds a particular public key to the name of the entity the certificate identifies.
Certificates help prevent the use of fake public keys for impersonation.
Only the public key certified by the certificate will work with the corresponding private key possessed by the entity identified by the certificate.