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.

See on RSA: http://www.rsa.com/rsalabs/node.asp?id=2124.

UPDATED: It's funny, that the old URL still on. It's a pity, that it shows absolutely another content as more as 5 years before. RSALabs had nice list of all PKCS standards with good overview. The new web-site have a little close information (currently) under the location (see here, here and so on).