What is a 'Hash Table' and how does it handle 'collisions'?

Hash function converts a key (e.g., AccountNumber) to an array index. Average O(1) for insert, delete, search  and it degrades to O(n) for worst case with many collisions. Collision resolution is done by Chaining in which each bucket holds a linked list of all keys mapping to that index (simple, handles high load factors). Open Addressing means on collision, probe for the next available slot (linear probing, quadratic probing, double hashing). Load factor (n/m) determines performance. Hash tables power Python dictionaries, Java HashMaps, database join operations, and caching systems.