This is how the GPIB language interfaces work:
The Declaration File contains NI-488 and NI-488.2 function prototypes and various predefined constants associated with the language interface. You must include the Declaration File with your GPIB Program Code, in order to make your GPIB application.
The Language Interface is a set of functions that call the GPIB Driver with the requested operation. For example, if you write a message to your GPIB instrument, you might use the ibwrt function. This function is declared in the Declaration File and defined in the Language Interface.
The GPIB Driver is the software that controls the GPIB hardware. It abstracts the low-level calls that make the board work, so that you can use the Language Interface to control the board. (Otherwise, you would have to learn register-level programming just to talk to your instrument!)
The GPIB Interface is the actual GPIB hardware, either installed in your computer or as an external controller (for example, GPIB-ENET).
Please note that the word interface is used quite often in GPIB. A language interface is an interface between a programming language and the GPIB driver. The GPIB driver is an interface between the GPIB program and the GPIB hardware. The GPIB hardware is an interface between the computer and the instrument.