DBTNG is the new database abstraction layer that makes connecting to and querying the database much easier in Drupal 7 than it was in the previous version of Drupal. DBTNG is an acronym for Database Layer: The Next Generation.
DBTNG was built with the intention of making it easier to support additional types of databases. Prior to Drupal 7, it was theoretically possible to create a Drupal-compliant compatibility layer to support any database. However, this required quite a lot of custom code and each custom module typically needed to add customized SQL statements to support multiple databases. In practice, this meant that Drupal only fully supported MySQL and PostgreSQL. Most contributed modules only supported MySQL although some also supported PostgreSQL. Site administrators who wanted to use a different database needed to do a great deal of work to migrate modules or build their own modules with support for their database of choice.
Some of the other key goals for...