The .der file extension is often used to refer to a file in the Distinguished Encoding Rules (DER) format. DER is a standard syntax for storing and transmitting data that is typically used to store X.509 certificates and certificate revocation lists (CRLs).
SSL stands for Secure Sockets Layer. It is a protocol that is used to establish secure connections between clients and servers over the internet. SSL uses a combination of public-key and symmetric-key encryption to secure the transmission of data.