Cryptographic Algorithms
There are two classes of key-based algorithms, symmetric (or secret-key) and asymmetric (or public-key). The difference is that symmetric algorithms use the same key for encryption and decryption (or the decryption key is easily derived from the encryption key), whereas asymmetric algorithms use a different key for encryption and decryption, and the decryption key cannot be derived from the encryption key.
Symmetric or private key algorithms are a very secure solution for an individual or small organization, but ineffective on a larger scale. Common symmetric algorithms are RC4 and DES.
Public key systems allow a cryptosystem to be maintained by a group of users. These systems rely on one-way functions, functions which are easy to calculate but hard to invert or reverse without prior knowledge. Each user has a private key that is kept secret and a public key that is publicly known. The best known public key system is RSA.
Many modern cryptographic protocols use a combination of public-key cryptography and symmetric cryptography to obtain the benefits of both: public-key algorithms to exchange a symmetric key, and symmetric algorithms to quickly encrypt or decrypt data.