Greenware is software distributed under the condition that the user does something to help the environment. The term "greenware" is a variant on shareware and freeware. Greenware can be distributed free or for charge. In either case the author expects the user to do something "green". That can be a special action like replacing all standard light bulbs with CFLs or a more general application to live more ecologically.
A more general approach than greenware is careware, which distributes software in a way that benefits a charity.
Greenware can also refer to a combination of computer hardware, software and services, which enables user to minimize the environmental impact of using the computer and lower TCO (Total Cost of Ownership) at the same time.
Examples
The Mac-software producer Mr. Fridge Software is distributing all their software under a so called "Green-License" which obliges the user to do something against Climate Change or Pollution. Although some suggestions are made, the License leaves it up to the user what actions he wants to take.