To simplify @horatio's above answer:
When you choose a Selection tool (Marquee, Lasoo, Quick Selection tool)
you will find options for adding/substracting/intersecting with an existing selection in the Tool Options Bar(up).
In this screenshot I am first(#1) creating a simple sqare selection with the "New selection" option of the "Rectangular Marquee Tool".
After I have the existing selection I am substracting a part from it using the "Substract from selection" option of the "Rectangular Marquee Tool".
I hope this clears any confusion about my answer. Good luck!
UPDATE - 25/02/2013**:
What the asker, Zak833 is actually looking for is something like the Pathfinder Tool in Illustrator, but for selections not for Shapes... he is trying to use the Selection Tool in a matter that the Shape Tool usually works,
A kind of workaround:
First work with the shapes you are trying to alter then recreate the selection from the shape.
Since Photoshop CS6 there are some simmilar options when creating a new shape(see screenshot). So your workaround here would be something like this:
Again, first alter/modify your shape then remake your selection from that new modified Shape.
** (needs editing and clarifying)