Yes — a layer mask is the standard way to do this in Photoshop CS5.

Basic workflow:

1. Put the foreground/subject on its own layer or make a selection of it.
2. Add a layer mask to that layer.
3. Paint on the mask: white reveals the layer, black hides it, and gray gives partial transparency.

If you don’t already have a precise selection, start with the Quick Selection tool to roughly select the subject. Then use Refine Edge to improve the boundary, especially around soft or complex edges like hair or fur. In CS5, Refine Edge can help detect and clean up the transition between subject and background.

Once the mask looks right, you can edit the foreground layer separately from the background without permanently erasing anything. That’s the main advantage of masks: they’re non-destructive and easy to tweak later.

For learning, search specifically for “Photoshop CS5 layer mask tutorial” and “Photoshop CS5 Refine Edge tutorial.”