A columnar database is a type of database management system that stores data in columns rather than rows. This structure allows for efficient data retrieval and analysis, especially for large datasets. By organizing data in columns, it can quickly access specific attributes, making it ideal for analytical queries and reporting. Columnar databases are particularly useful in scenarios involving big data and business intelligence, where fast read performance is crucial. Popular examples include Apache Cassandra and Google BigQuery. These databases excel in handling complex queries and aggregations, providing significant performance advantages over traditional row-based databases.