In Symmetric Cryptography (also: secret key cryptography), an algorithm is used to scramble the message using a secret key in such a way that it becomes unusable to all except the ones that have access to that secret key. The most widely known symmetric cryptographic algorithm is DES, developed by IBM in the 70ies. It uses a key of 56 bits and operates on chunks of 64 bits at a time. The problem with DES is the short key length, making it possible for someone, that doesn’t have access to the private key, to try all possible keys. DESede provides a significant improvement with the key being 3 times as large.
Symmetric cryptography works very well if encryption and decryption are eventually performed by the same party. But if there are 2 or more parties involved, there is the question of how to exchange the secret key without anyone spying. Look at asymettric cryptography for a solution.