You can't do this using the standard iPad Maps app, but there are several third-party apps that will allow you to download maps for use off-line. Generally it depends upon which country you are in. I'm in the UK, and there's an app UK Maps that enables you to do that. Also, do you want topographic maps or 'sat nav' style maps - for that have a look at Navigon and TomTom - those apps are available in most countries. If you let us know the country you're in, I'm sure they'll be a Member who can help you.
Tim