In principal its very easy,all you do in something like photoshop is put each photo on a separate layer on a white/coloured background move them to suit the composition and then flatten the layers.
Note I said in principal,in practice it is not that easy to achieve the look in the photo you have posted.
You have to either photograph the object and the background under exactly the same lighting conditions so when you put one on top of the other they blend seamlessly together or you have to extract the image and paste it on to the background and that is not as easy as it sounds, it is very dependant on the colour separation between the object and the background and the edges of the object you want to extract(complex edges like hair are difficult)
If you google 'green screen' and 'edge selection in photoshop' that will give you some idea.
There are some good tutorials about edge selection using CS5 and the refine edge tool but they do use a best case scenario,usually on a grey background which makes it look easy ( believe me it ain't)
Best of luck with it.