The .pem file extension stands for Privacy-Enhanced Mail, although its usage has expanded significantly beyond its original email security context. A PEM file is fundamentally a container format used to store cryptographic keys and certificates, typically in Base64 encoded ASCII text format. This format is highly prevalent in the world of Public Key Infrastructure (PKI) and secure communications, such as SSL/TLS for websites. The structure often begins with a header line like '-----BEGIN CERTIFICATE-----' or '-----BEGIN PRIVATE KEY-----' and ends with a corresponding footer. Because it is text-based, it is easily readable and transferable across different systems and applications that handle encryption, digital signatures, and authentication. PEM files are commonly used by web servers (like Apache and Nginx) to store server certificates and private keys required to establish secure HTTPS connections. They can contain X.509 certificates, private keys, Certificate Signing Requests (CSRs), or a combination thereof, distinguished by the specific header/footer tags present within the file.