Collage is a type of art which combines pieces of diverse materials such as magazines, types of paper, fabric, paint and photographic material into one composition. A good starting point in understanding what collage is, might be to look at some of these artists who pioneered the idea of collage:
Kurt Schwitters, Picasso, Matisse, Hannah Höch
There are many modern day collage artists who are using a range of media and materials to create a collage. Many collage artists now use Photoshop to manipulate images and photographs digitally. There really seems to be so much scope with this medium which makes it very exciting.