basically, the key size represents the maximum possible security that the cryptographic algorithm can provide, since the slowest possible method would be brute-force. however the provided security may be lower if there is a lower know attack complexity.http://en.wikipedia.org/wiki/Key_size
some good reads on crypto are here: http://www.cryptopp.com/wiki/Recommended_Books
some good APIs for crypto (C & C++): CryptoPP, Botan, and (perhaps obviously) OpenSSL.
other than that, wikipedia is decent for understanding crypto terminology.