A collage is a composite image ie Photoshop layers were use. One might say every every Photoshoped image is a collage. Its the way layer are put together and blended. I put together a Photoshop package I call "Photo Collage Toolkit" for making collages using PSD templates and Photoshop Scripts. You can even add in an Action or two. I supply the rules you need to follow when you create your PSD template files and supply all the scripts you needed to populate your templates with your images.
Link to the Photo Collage Toolkit documentation and download: http://www.mouseprints.net/old/dpr/PhotoCollageToolkit.html
A example of a picturless psd template populated with my wife's pictures the other day click on the below image for a Video Demo: