Photomontage is the combing of two or more images to create something completely new and unique. The two ways to do this are through analogue or digital techniques.
Analogue Photomontage
Analogue techniques are used to combine images by hand using a knife/scissors, your hands and glue etc. For example, you could tear up an image and stick parts of it onto another to create a rough and damaged effect. You can then take a photo of your final outcome and re-print it to create a perfect undamaged image.
Digital Photomontage
Digital techniques combine images using software like Adobe Photoshop. These images can look similar to analogue photomontages but they eliminate the mess and retaking of the image when finished.