You want to represent cryptographic data such as public keys or certificates in a plaintext format, so that you can use it in protocols that don’t accept arbitrary binary data. This may include storing an encrypted version of a private key.
The PEM format represents DER-encoded data in a printable format. Traditionally, PEM encoding simply base64-encodes DER-encoded data and adds a simple header and footer. OpenSSL provides an API for such functionality that handles the DER encoding and header writing for you.
OpenSSL has introduced extensions for using encrypted DER representations, allowing you to use PEM to store encrypted private keys and other cryptographic data in ASCII format.
Privacy Enhanced Mail (PEM) is the original encrypted email standard. Although the standard is long dead, a small subset of its encoding mechanism has managed to survive.
In today’s day and age, PEM-encoded data is usually just DER-encoded data with a header and footer. The header is a single line consisting of five dashes followed by the word “BEGIN”, followed by anything. The data following the word “BEGIN” is not really standardized. In some cases, there might not be anything following this word. However, if you are using the OpenSSL PEM outputting routines, there is a textual description of the type of data object encoded. For example, OpenSSL produces the following header line for an RSA private key: ...