Encryption, the transformation of data into a form that prevents anyone unauthorized from understanding that data, is a fundamental technology that enables online commerce, secure communication, and the protection of confidential information.
Encryption algorithms are the mathematical formulae for performing these transformations. You provide an encryption algorithm with a key and the data you want to protect (the plaintext), and it produces an encrypted output (the ciphertext). To read the output, you need to feed the key and the ciphertext into a decryption algorithm (sometimes these are identical to encryption algorithms; other times they are closely related but different).
Encryption algorithms are designed so that performing the decryption process is unfeasibly hard without knowing the key.