ND filters reduce the amount of illumination, which you should be able to
accomplish by lowering the ISO, and/or using a smaller aperture (or higher F
stop).  Keep in mind a smaller aperture will also increase your depth of
field, which is usually desirable with landscapes.  A polarizing filter
reduces glare from objects like glass, water, etc.

Bill Crocker

Not the same thing at all.
All a ND filter does is add density to the image. I.e., it darkens the image
before it hits the lens, for those scenes that are too bright for your
f-stop/shutter-speed/ISO combination.

A polarizing filter does something quite different.

A polarized filter will block light rays that are perpendicular to the
foil grid while light rays from other directions may only be partially
suppressed. Depending upon the direction of reflected light, the color
and contrast of a scene may be altered. A polarizing filter should not
be indiscriminately used. There are times when such a filter will be
very helpful and there are times when it should not be used. It pays to
experiment and learn.

Neutral Density (ND) filters uniformly reduce the amount of light of a
scene which allows high speed film to be used in bright light areas.
They may also be used to improve depth of field by increasing the
aperture and exposure time beyond what normally would be used or allow
the scene to be photographed within the capability of the lens/camera.
Graduated ND filters may be used to balance the light rays of a scene.
Neither type filters, ND or Graduated ND, may be used to alter the color
or contrast of a scene.

I believe you mean an ND graduated filter, not just a plain ND. The
short answer is that the ND grad will darken the sky evenly, including
clouds, sunbursts, rays, etc and leave the ground normally exposed. You
must experiment a bit with exposure to get it right, and you can get
different strengths from 2X to 16X density in the sky, as well as
different graduations from sharp border to very gradual shading. Serious
ND grad users often have half a dozen different ones, plus a holder
which allows them to be rotated and slid around to get the graduated
zone in the right place. They can also be used with a polariser.

A polariser will transmit or reject polarised elements in light. These
can include areas of the sky which we see as blue, mainly to the sides
of the sun position, not right next to the sun or with the sun dead
behind the camera. They also include details with reflective sheen (waxy
foliage, leaves, grass, wet roads, polished wood, polished car
paintwork, reflections in glass at around 40 degree angle, reflections
off water under similar conditions, and of course reflections off human
skin and hair). Using a polariser mean rotating it until the main
element you want to change is visually altered. However, there are
conflicts. When the sky is darkened (and it only works on blue skies,
having no effect on the density of clouds or dull day skies) you may
also be making water lose sparkle, or a portrait appear like cardboard
because the gloss is lost from skin and hair. Alternatively, you may end
up boosting these qualities.

ND grads do not need an increase in normal exposure, polarisers may need
up to 2 stops as they include a significant overall ND density.

For autumn/fall colours, the polariser is essential, especially with
water, lakes, forests. Autumn skies are highly responsive to polarising
and fall leaf colours are emphasised by cutting the sheen from many of
the angled leaves (since they are orientated at random, it works almost
regardless of polariser rotation).

David

Insults to all-hehhe

bruce is my bro's dog :D

Producing vocabulary that has relevance to 9th grade education
will help the other respondents :)

go with a ND grad- answering and side steping all the same :)