A Hashtable is a data structure that stores key-value pairs, allowing for efficient data retrieval. It uses a hash function to compute an index into an array of buckets or slots, where the desired value can be found. This enables quick access to data, typically in constant time, O(1), for lookups, insertions, and deletions. In a Hashtable, each key must be unique, and if two keys hash to the same index, a collision occurs. Various methods, such as chaining or open addressing, can resolve these collisions. Common programming languages like Java, Python, and C# provide built-in support for hashtables, making them easy to implement.