PEM Format
The PEM format is the most common format that Certificate Authorities issue certificates in.
PEM certificates usually have extentions such as .pem, .crt, .cer, and .key.
They are Base64 encoded ASCII files and contain "-----BEGIN CERTIFICATE-----" and "-----END CERTIFICATE-----" statements.
Server certificates, intermediate certificates, and private keys can all be put into the PEM format.
Apache and other similar servers use PEM format certificates.
Several PEM certificates, and even the private key, can be included in one file, one below the other, but most platforms,
such as Apache, expect the certificates and private key to be in separate files.