The database connectivity standard is an established fact, with its most successful commercial implementation being Microsoft ODBC 3.0 interface. There are dozens of ODBC drivers for all types of data sources ranging from RDBMS to flat text files, available on virtually every platform — Microsoft Windows, various Unix flavors, and Mac OS, to name just a few.
The CLI/SQL standard establishes a set of functions that are used by applications in a uniform fashion to access data sources and perform data-related tasks.
All communication between the application and the data source is conducted through ODBC drivers, which are loaded and unloaded on demand by the Driver Manager. The ODBC (CLI/SQL) drivers work in a way similar to a live interpreter — it speaks a common language (ODBC functions set) to the application (through the Driver Manager) and also speaks native RDBMS dialect.
There are a number of emerging technologies for defining the application-RDBMS interaction. The most popular are Java Database Connectivity (JDBC), Microsoft Active Data Objects (ADO), and ADO.NET (an object-oriented interface to the OLEDB API or ODBC API), in addition to the new "native" .NET providers.
Every RDBMS vendor has its own version of Call Level Interface: Oracle's Oracle Call Interface (OCI), IBM DB2 UDB Call Level Interface (CLI), and Microsoft DBLIB. Some of these are becoming increasingly obsolete, although they provide the maximum performance in terms of the raw speed.