Hi Doug,
It usually involves selecting and seperating the subject from the back ground. The next step is to place the seperated subject onto another backgrond.
Selecting in PS can be simple or very involved and there are many ways. The simplest I find is the lasso too.
All this is done on seperate layers.
example: I want to seperate my granddaughter and place her on a black ground.
1. the original picture on layer 1
2. using the lasso ( magnetic lasso ) tool make a selection of the subject..the child. you can refine the selection edges by going to 'select' menu and ' refine '.
pressing option-j ( mac ) and cntrl-j ( windows ) you can get the selected subject on a seperate layer
above layer 1. call it layer 2. like so..
3. if you now place a layer..3.. in between layer 1 and 2... a black background
you would have a different background for the child.
darn..i realize i closed the file without saving the final background. but you get the idea.
Thanks to Mariam for allowing me to demonstrate on her! total time less than 5 minutes.
Hope this helps.