A hash table, or a hash map, is a data structure that 
associates keys with values. The primary operation it 
supports efficiently is a lookup: given a key (e.g. a 
person's name), find the corresponding value (e.g. that 
person's telephone number). It works by transforming the 
key using a hash function into a hash, a number that is 
used as an index in an array to locate the desired location 
("bucket") where the values should be.

1.Hashtable is a class which falls into System.Collections 
namespace. 
you can create object for hashtable. For example,
         Hashtable hash = new Hashtable();

2.Key and value should be added to hashtable. For example,
         hash.Add(1,"God");
         hash.Add(2,"Human");

3.The key, value will be sorted automatically based on the 
Hashcode generated[this operation is taken care by CLR].

4.GetHashCode() method is used to get the generated hashcode

5.While retrieving the hashtable values, you can use the 
key tp retrieve the values. For example
    foreach (DictionaryEntry item in hashTable)
    {
      Console.WriteLine("Key is {0}, Value is 
{1}",            item.Key,item.Value);
    }
NOTE: if you add the same key into the hastable, it will 
not throw any compliation error, but during runtime, it 
will throw "ArgumentException" error.