PEM (.pem, .crt, .cer, .key)
Base64 ASCII format enclosed between -----BEGIN ...----- headers. Standard for Nginx, Apache, Linux, and OpenSSL.
PKCS#12 (.pfx, .p12)
Password-protected binary format storing certificate, private key, and intermediate chain in one file. Standard for Windows IIS, Azure, and Tomcat.
PKCS#7 (.p7b, .p7c)
Base64 format storing certificate chains and intermediate CAs without private keys. Used in Windows Keystores and Java Tomcat.
DER (.der, .cer)
Binary encoded form of certificates. Common in Java platforms and some legacy banking hardware.