Symmetric-key cryptography is a term used for cryptographic algorithms that use the same key for encryption and for decryption. The key is usually called a "symmetric key" or a "secret key".
This is usually contrasted with public-key cryptography in which keys are generated in pairs, and the transformation made by one key can only be reversed using the other key.
Symmetric-key algorithms should be secure when used properly and highly efficient, so they can be used to encrypt large amounts of data without having a bad effect on performance.
Most symmetric-key algorithms currently in use are block ciphers: this means that they encrypt data a block at a time. The size of each block is fixed and determined by the algorithm: for example AES uses 16-byte blocks. Block ciphers are always used with a mode, which specifies how to securely encrypt messages that are longer than the block size. For example, AES is a cipher, while CTR, CBC, and GCM are all modes. Using an inappropriate mode, or using a mode incorrectly, can completely undermine the security provided by the underlying cipher.