Cryptography is principally divided into symmetric and asymmetric encryption. Symmetric encryption refers to encryption in which the key is either pre-shared or negotiated. AES, DES, and Blowfish are examples ofalgorithms used in symmetric encryption.
Most savvy computer users are familiar with WEP, WPA, or WPA2, which are security protocols employed in Wi-Fi connections. These protocols exist to prevent the interception and manipulation of datatransmittedover wireless connections (or, phrased differently, to provide confidentiality and integrity to wireless users).Routers now often come with the wireless password printed on them,and this is a very literal example of a pre-shared key.
The algorithms used in symmetric encryption are often very fast, and the amount of computational power needed to generate a new key (or encrypt/decryptdatawith it) is relatively limited in comparison to asymmetric encryption.