PDO is referred as PHP Data Objects. It is a lean and consistent way to access the database. It helps the developers to write portable code in the easier way. It is like a data access layer that uses a unified API.
Drupal provides a database abstraction layer, which helps the developer to work easily with multi-database servers. It is used to preserve the syntax and power of SQL and to work with complex functionality. It provides a defined interface for dynamic queries with using security checks and good practices. This process is developed at the top of the PDO database API.