Filters:-
UV filter - can be left in place all the time and will give some protection to the lens against stray fingerprints, dust, etc.
Polarising filter - use it to cut glare, reflections, and turns to help darken the sky at some angles (don't use on wide angle lenses as it looks odd).
Neutral Density Graduated filters - usually square and require a special holder, but very useful for landscape photography as they cut light from the sky so you don't underexpose the ground or overexpose the sky.
Half the filter is clear, and the other half slowly gets darker as you approach the top.
I shot this using an ND grad:-
The sky's well exposed and so is the ground. Without that filter either the sky would be pale and washed out or the ground detail too dark.
I use the
Cokin system but there are others.
You can't duplicate a polarising filter in photoshop, or a ND grad (short of using HDR techniques).
Hope this helps.