Hashing

store key or key/data record pairs in an array, called a "hash table"
make the hash table size comparable to the number of actual keys that will be stored in it --- typically much smaller than the number of possible key values
given a key, compute the index of that key’s location in the array using a fast O(1) function, called a "hash function"

Since the hash function is O(1), a hash table has the potential for very fast find performance (the best possible!), but...
... since the hash function is mapping from a large set (the set of all possible keys) to a smaller set (the set of hash table locations) there is the possiblity of collisions : two different keys wanting to be at the same table location