Windows 98 / Windows 95 / Windows 2000 / Windows 98 SE / Windows XP / Windows NT / Windows ME
Visualboy Advance is easily the best emulator created for Gameboy Advance games, hands down. One of those rare pieces of software that people use over a decade after it was first released, VBA is the gold standard that other Gameboy emulators were judged by.
The idea of an emulator is simple: A computer program that acts like another computer program. In this case, VBA emulates the software that runs a Nintendo Gameboy Advance. You point it at a file (called a ROM file) that contains the contents of a cartridge, and the emulator shows you exactly what the hardware would have put on the screen.
In other words, it lets you play Gameboy and Gameboy Advance games on your computer.
Good emulators provide options you can't get on the original hardware. In this case, VBA offers a wide array of options, including the ability to emulate additional hardware (such as the Super Gameboy, the Gameboy Printer, GameShark Advance, and turbo-fire buttons), the ability to debug (or 'hack') your games, and the ability to record sound and video files directly from your gameplay. All of this is accomplished through an easy-to-use interface.
There's a reason why this ten-year-old piece of software can often be found on state-of-the-art hardware; it's often the first thing people install on their new computers. Give it a try, and you'll find yourself making sure you always have a copy of it for when you want to remember some retrogaming goodness.