Montage in the photographic world is the process of creating a single image from numerous smaller images. The composition may be created by juxtaposition or by merging multiple images to conceal boundaries. However, in either case, the result is a single image and regarded as somewhat more formal in character than a collage. The image below was created for a family history book cover and incorporates numerous images taken from different generations of the same family.
A quick search of internet photography sites shows that there is considerable confusion between the definitions of montage and collage. Indeed the two categorizations do seem to overlap to some extent - perhaps an absolute differentiation is unimportant.
However, a collage is generally regarded as a sequence of separate images assembled on a background in an aesthetically pleasing manner. The individual images typically have a unifying theme, subject or characteristic. The final work features a number of distinctly separate images combined in an informal, or apparently informal, manner.