DER Distinguished Encoding Rules is a key file format for cryptographic data.
It's the binary ASN.1 encoding of the data (not in plain text otherwise it's a PEM.
pem is the base64 encoded version of the DER formated data with additional header and footer lines to be transported via e.g. E-mail
to Privacy-Enhanced Mail (PEM) (OpenSsh key format)
openssl x509 –in file.der –inform DER –out file.pem –outform PEM