DOSBox is a DOS-emulator. You use it when you want to play your classic DOS/PC games on newer machines. A useful tutorial on how to download, install and run games with DOSBox is found on their own website, Basic Setup and Installation of DOSBox. They also have a special information section about Vista, DOSBox and Windows Vista.
If you are a mac user you should take a look at http://boxerapp.com. It's a GUI based dosbox frontend. Installing a dos game on mac is now just a question of drag and dropping.
To read more, visit the official DOSBox website. Any questions can be posted in the forums and will be quickly answered by the community here, dosspot Forums.