Parity is an error detection technique. Parity works by adding one extra bit - the parity bit - to a given word or field.
Under even parity, the parity bit is set such that each group of bits always contains an even number of ones and zeros. Under odd parity, the opposite is true - each group of bits will have an odd number of ones and zeros.
Parity works well only if bit errors are rare and tend to affect only single bits. In the event that bit errors occur in clusters, parity is less effective, because any even-numbered amount of bit errors will offset each other and not be detected through a parity check.