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. It started in the beginning of the 20th century. 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.