Symmetric-key algorithm
Symmetric-key algorithms are very important because they are faster on computers than the other kind:public-key algorithms. In public-key cryptography (asymmetric-key cryptography) the key for encryption can be given to the public with no problem, and everyone can send you secret messages. The key for encryption is "open" because, in practice, it cannot be used to get the key for decryption. This is very useful, but public-key cryptography algorithms are very slow on computers, so they are only used to send a secret key. Then symmetric-key algorithms are used for everything else because they are faster.