PostgreSQL, often referred to as Postgres, is an open-source relational database management system (RDBMS). It is known for its robustness, extensibility, and adherence to SQL standards. PostgreSQL is designed to handle large amounts of data and support complex queries and transactions.

Here are some key features and characteristics of PostgreSQL:

1. Relational Database: PostgreSQL follows the relational database model, organizing data into tables with rows and columns. It provides ACID (Atomicity, Consistency, Isolation, Durability) compliance, ensuring data integrity and reliability.

2. SQL Support: It supports the SQL (Structured Query Language) standard, allowing you to perform a wide range of database operations such as creating, modifying, and querying data using SQL statements.

3. Extensibility: It offers a rich set of data types, including built-in and user-defined types. It also supports custom functions, operators, and procedural languages, such as PL/pgSQL, PL/Python, PL/Perl, and more. This extensibility allows developers to tailor the database to specific application requirements.

4. Concurrency and Transaction Control: It provides robust concurrency control mechanisms to handle simultaneous database access by multiple users. It supports multi-version concurrency control (MVCC), allowing transactions to proceed independently without blocking each other. Additionally, PostgreSQL provides transaction isolation levels to control the visibility and consistency of data during concurrent operations.

5. Replication and High Availability: It offers built-in replication features that allow you to create copies of the database for backup, scale-out, or fault tolerance purposes. It supports both asynchronous and synchronous replication methods to ensure data consistency across multiple servers.

6. Advanced Features: It includes advanced database features such as table inheritance, views, triggers, stored procedures, full-text search, JSON and XML support, spatial data capabilities, and more. These features enable developers to build complex and powerful applications.

7. Community and Ecosystem: It has a vibrant and active open-source community that continuously develops and improves the database. It has a rich ecosystem of extensions, tools, and frameworks that enhance its functionality and make it easier to work with.

PostgreSQL is widely used in various domains, including web applications, enterprise systems, geospatial applications, analytics, and more. Its flexibility, scalability, and adherence to standards have contributed to its popularity among developers and organizations of all sizes.