pem is a de-facto file format called Privacy-Enhanced Mail. These are interchangeable file extensions for the PKCS#12 format.
Technically, PKCS#12 is the successor to Microsoft's PFX format, but they have become interchangeable. PKCS#12 files are archives for cryptographic material.
PKCS#12 (also known as PKCS12 or PFX) is a binary format for storing a certificate chain and private key in a single, encryptable file.
All SSL Certificates require a private key to work. The private key is a separate file that's used in the encryption/decryption of data sent between your server and the connecting clients.
A private key is created by you—the certificate owner—when you request your certificate with a Certificate Signing Request (CSR).
Basically, the default SSL file format used by apache web server is the PEM format. Whereas PFX files are used on MacOS and Windows systems to do export and import activities of private keys and certificates.