What for are the commonly used PKCS-Standards: PKCS#7, PKCS#10 and PKCS#12?
PKCS#7 lets you sign and encrypt generic data using X.509 certificates. Also PKCS#7 format can be used to store one or more certificates without private keys (private keys can be put as a data payload and encrypted this way).
PKCS#10 defines format for certificate requests.
PKCS#12 provides a container for one or several certificates with private keys.