Given a character string, a hashing algorithm will generate a unique numeric value for that string. This numeric value is known as the string's hashcode. No two, non-identical, strings should have the same hashcode (given a decent hashing algorithm!!). This is the principal behind hashtables etc. The string used as the key is converted to a hashcode and is used as an index into the data collection. There are lots of books on such algorithms if you are interested - have a look at 'Fundamental Algorithms', for example, by Donald Knuth.

Regards.