Basically GPIB allows you to control instruments like Power Supplies, Signal Generators, Oscilloscopes, Signal Generators, etc. You need to install or connect a GPIB interface card (PCI, PCMCIA or USB) to your PC and install the corresponding GPIB driver.
VISA is a standard API that sits on top of the GPIB driver and it allows you to use the same programs to control your instruments regardless of the type of GPIB card that you have installed in your PC (most cards are made either by National Instruments or by Agilent/Hewlett-Packard).
The cool thing about this library is that it opens AutoIt to a whole new kind of automation (instrument automation). Normally you would need to use some expensive "instrumentation" environment like Labwindows/CVI, LabView or Matlab to automate instruments but now you can do so with AutoIt. The only requirement is that you need a VISA compatible GPIB card (all cards that I know are) and the corresponding VISA driver must be installed (look for visa32.dll in the windows\system32 folder). 2b1af7f3a8