what is the difference between arrays and linked list?

Array is fixed length and Array is a simple sequence of numbers which are not concerned about each-others positions but linked list is variable length In array values are accessing easy but linked list is some time taken process because search either forward or backward In array updating operations are time taken comparing with linked list Array is not grow-able and linked list is grow-able

What is hashtable?

A hash table is a generic data structure providing for fast and random retrieval of objects. It is sometimes called a map or a dictionary (although the implementations may differ). Its a key-value pairs. The tenet of a hash table is that the location of an object in the data structure is computed from the key associated with the object when it is stored into the hash table. For that reason, retrieving an object from a hash table is very fast. Of course, the key associated with the object must be unique.