A cipher which uses a different encryption key every time, as long as the message. The key is XOR'ed with the message to render the cipher text which can then be XOR'ed with the same key to get the plain text.
A cipher which uses a different randomly-generated, pre-shared key stream of the same size as the message. Commonly the plaintext is XOR-ed with the key stream to generate the ciphertext. Similarly XORing the ciphertext generates the plaintext.
Modular addition / subtraction may be used instead of the XOR operation. As long as the key stream is perfectly random, the OTP is considered provable secure; in practice the stream is never perfectly random.
Another issue with the one-time-pad is that the size of the key stream must have at least the same size as the message. This makes it impractical for many use cases.