SHA-1 is the first version in the Secure Hash Algorithm (SHA) family that was made widely popular. Usage of this algorithm with an input value provides a 160 bit hexadecimal value with hash properties (cannot be reversed to find the original value ). This method is used to maintain integrity of data as different input values always create different hash values and therefore making a duplication of hash difficult. SHA-1 was an improvement over a previous hashing algorithm called MD5.
For more details check the FAQ for this algorithm.