Hashing

What is hashing.How hash functions for numbers and strings cab be decided.

Useful Links:
http://www.personal.kent.edu/~rmuhamma/Algorithms/MyAlgorithms/hashTable.htm
http://coding-geek.com/how-does-a-hashmap-work-in-java/

Discuss Open hashing and closed hashing concepts

Separate Chaining:
http://quiz.geeksforgeeks.org/hashing-set-2-separate-chaining/
Open Addressing:
http://quiz.geeksforgeeks.org/hashing-set-3-open-addressing/