Lenses are arguably more important than Camera bodies. Many a professional will tell you it's better to invest in good glass over expensive cameras if you have to choose between the two (and you may have to choose -- as both DSLR's and quality DSLR lenses are expensive).
Lenses Fall into three broad categories:
Wide Angle -- a lens with a short focal length that takes in a wide view (typically 10mm-24mm)
Normal -- show what or close to what the human eye sees (30-60mm)
Telephoto -- Magnify the object (typically 70mm+)
Other Specialty Lenses:
Prime -- a lens with no zoom feature; typically sharper than zoom lenses
Fisheye -- offer a distorted wide view
Zoom -- a lens that can change its focal length
Macro Lens -- lenses that have a close up mode for close up photography
Now the lens that MOST landscape photographers will utilize would be Wide Angle because you can capture the entire landscape scene more easily. Wide Angle Lenses give you an expensive picture.
You can further divide Wide Angle into two sub categories:
Super Wide Angle (less than 15mm)
There are two types of lenses you'll find in wide angle: fish-eye and rectilinear lenses.
Fish-Eye lenses create a curved effect to the composition whereas Rectilinear lenses render the final composition in straight rather than curved lines. Super-Wide Angle lenses are usually vastly expensive and whilst creating a dramatic optical effect, it is the choice of landscape and execution of the image that matter beyond all else.