A collage is both an artwork made from objects that are glued down onto a surface, such as a piece of paper, canvas, or board, and the technique of making such an artwork. The objects can be whatever you wish, for example tissue paper, wrapping paper, newspaper pages, magazine pages, cardboard, foil, metal, plastic, fabric, wire, photographs, found objects such as shells, feathers, or stones, and 'rubbish' such as broken toys or appliances. You can even cut up paintings that haven't worked or paintings you deliberately created to be collage elements. The heavier the objects, the stronger the support needs to be.
The issue of whether the materials you are using are archival is one to be considered if you with a collage to last. Use archival glue, especially if you're working on paper. Don't nick your kids' gluestick; it's likely to stain with time.
Newspaper and magazine images are not printed on acid-free paper, but their fading and changing color can be part of the work. Pasting them onto primed canvas or wood, rather than paper, and then varnishing them may help preserve them.
Prominent artists who made collages include Picasso, Braque, and Matisse.
Papier collé is a French term meaning "pasted paper". It's a type of flat collage in which the artist glues down flat pieces (paper, newspaper, fabric, etc) that are the shapes of objects, rather than building up shapes with collage pieces. It's believe papier collé was first done by the artist Georges Braque, in his 1912 painting Fruit Dish and Glass.
Collage differs from assemblage in being a two-dimensional artwork, whereas assemblage is three-dimensional sculpture, though the distinction is sometimes burred. Collage is also distinguished from decoupage, which is a crafts technique that involves pasting down images.