Nagware is a software distribution model where distributed program is free and has all its features available, but the user will be asked to give the developers money. Some nagware is pretty unobtrusive, like merely adding a donate button to the user interface, but some is very obnoxious, popping up messages with long delays asking the user to pay in order to eliminate them. This is very similar to adware, only, the ad is for the program itself.
I don't have an ethical problem with nagware in any form. You're being allowed to use a product for free which took someone a lot of effort to make, so an annoying message begging for money is hardly an issue. I'm completely fine with nagware when it's unobtrusive, like in a menu or about dialog. When the nag becomes more obtrusive, like the delay used in early versions of WinZip, it usually causes me to find a different free product, which led me to 7-Zip.