PEM is a widely used format for security certificates. Syntax and content are defined by X.509 v3 standards for digital certificates and its specifications are defined in IETF RFC 5280.
A PEM certificate is a base64 (ASCII) encoded block of data encapsulated between two header lines:
-----BEGIN CERTIFICATE-----
and
-----END CERTIFICATE-----
While, Private Keys use the identifier:
-----BEGIN PRIVATE KEY-----
and
-----END PRIVATE KEY-----
The most used file extensions for PEM files are .pem, .crt, .ca-bundle but you may find it under other extensions too.