Hash tables are also called hash maps, maps, unordered maps, or dictionaries. A hash table is a structure that maps keys to values. This makes them extremely efficient for lookups because if you have the key, retrieving the associated value is a constant-time operation.
Hash tables have fast lookups (O(1)) on average. However, in the worst case, they have slow (O(n)) lookups. The slow lookups happen when there is a hash collision (two different keys hash to the same index).
Hash tables have fast insertions (O(1)) on average. However, in the worst case, they have slow (O(n)) insertions. Just like with the lookups, the worst case occurs due to hash collisions.
Hash tables have fast deletes (O(1)) on average. However, in the worst case, they have slow (O(n)) deletions. Just like with lookups and insertions, the worst case occurs due to hash collisions.