The process of making a collage is very well known and used by many people and the technique is very simple: Mix different materials to make a composite image.
Collage comes from the French word coller, which means to glue. The French word in turn comes from the Greek word kolla.
What is surprising is that although the technique is very old, the word collage itself did not come into use until around 1915-20 when it was coined by Georges Braque and Pablo Picasso to describe their compositions that included painted elements mixed with scraps of newspaper, bus tickets and other ephemera.