A TreeSet is a collection in Java that implements the Set interface, which means it stores unique elements without duplicates. It is based on a red-black tree, a type of self-balancing binary search tree, ensuring that the elements are always sorted in natural order or by a specified comparator. This allows for efficient retrieval, addition, and removal of elements. One of the key features of a TreeSet is that it maintains the order of its elements, making it useful for applications where sorted data is required. However, because of its underlying tree structure, operations like adding or removing elements can be slower compared to other sets, such as HashSet, especially for large datasets.