Hash is a algorithm used to generate a fixed length string from any given text which is usually shorter in length than the original text. This hash can be used to verify the integrity of the original data. It is also used in storing passwords. Hash algorithm quarantees the same hash value for a given text - meaning the hash value will not change if the hash is regenerated for the same piece of text.
What are the different hash algorithm?
MD5 is the most popular algorithm used on the web. SHA (various forms) have started gaining popularity and SHA512 is the most secure hash algorithm.