PostgreSQL is an advanced open-source relational database management system known for its robustness, scalability, and compliance with SQL standards. It supports complex queries, transactions, and a wide range of data types, including JSON and XML. PostgreSQL is highly extensible, allowing users to define custom data types, functions, and plugins. It is widely used in web applications, data analytics, and large-scale enterprise systems. Its focus on reliability and performance makes it a popular choice for modern developers.