The primary types of databases used today include relational databases, NoSQL databases, and in-memory databases. Relational databases, like MySQL and PostgreSQL, organize data into tables with predefined relationships. NoSQL solutions, such as MongoDB and Cassandra, are more flexible and allow for unstructured data. In-memory databases, like Redis, store data in RAM for rapid access and are often used for real-time applications. Each type serves different needs, so understanding their strengths helps in selecting the appropriate database for specific use cases.