PostgreSQL is a robust, open-source relational database management system (RDBMS) known for its advanced features, extensibility, and strong support for SQL and NoSQL querying. It offers high performance, ACID compliance, and supports complex data types, making it suitable for handling large-scale and mission-critical applications. PostgreSQL is highly extensible, allowing users to define custom data types, functions, and indexes. With its focus on data integrity, concurrency, and scalability, it is widely used for applications requiring complex queries, high reliability, and flexibility.