PKCS11
PKCS11 is an API to talk to smartcards.
PKCS15
PKCS15 is a way of storing data on the smartcard. PKCS15 defines the container structure on the smartcard.
Links
All the pkcs-standards are defined and described at the RSA-Website:
http://www.rsasecurity.com/rsalabs/node.asp?id=2141 or take a look at http://en.wikipedia.org/wiki/PKCS.