A computer projector pretty much takes the hassle of creating transparencies for overhead projectors. These machines put the screen of your computer up on the screen and make it easier for teachers to present materials for classes and it makes it big enough for everyone to read. You can go through powerpoints for notes in a lecture or show students how to access information on a website. It can also be used as a help for an art teacher for graphic design by showing students how to use different programs to make works of art.
http://www.zardec.net.au/keith/project.htm
