Public Key Cryptography Standards - PKCS - is a collection of public-key cryptography standards.
The PKCS #11 standard defines a platform-independent API to cryptographic tokens, typically used for hardware security modules or smart cards.
The PKCS#12 standard is a binary format for storing the server certificate, intermediate certificates, and the private key in one file, a PFX file (usually with th extension .pfx and .p12).