A HashMap is a data structure that stores key-value pairs, allowing for efficient data retrieval. It uses a hash function to compute an index where the value associated with a specific key is stored. This enables quick access to values, typically in constant time, making it ideal for scenarios where fast lookups are essential. In a HashMap, keys must be unique, while values can be duplicated. If two keys hash to the same index, a collision occurs, which is handled through various methods, such as chaining or open addressing. This structure is commonly used in programming languages like Java and Python for managing collections of data.