What's the difference between them?

They work differently

Basically, a polarizer is used for blocking light reflected off a surface, while an ND just makes the whole scene darker. Polarizing filters can enhance the color of the image while ND filters just block the light entering the camera. It does not change the overall color of the image.

ND filter:

An ND filter can block the light entering the camera sensor. It can help you to use slow the shutter speed or wide aperture in a harsh light. This is useful when a sufficiently long exposure time is not otherwise attainable within a given range of possible apertures (at the lowest ISO setting).

Polarizer (CPL filters):

One of the biggest frustration in landscape shooting is the lack of color. The quickest way to reduce the reflection of the image is to use a polarizing filter.

What does an ND filter do?

1:Aperture: use a shallow depth of the filed even in a bright light

2:Shutter speed: introduce slow shutter speed to get smooth water movement effect

3:Motion blur: help to capture the motion blur moving objects.

   With ND Filter and without ND filter

What does a CPL filter do?

1: Reduce the reflection and glare cast on non-metallic reflective surfaces.

（the light reflected from a metal surface and the polarizer has no effect on it)

2:Enhance the contrast of the image. Saturate the colors and enhance the image quality.

By adjusting the POLARIZER the reflection can be reduced, thereby the image quality and clarity can be improved. The best angle of using a polarizing filter is at a 90-degree angle to the sun.

3:Reduce the glare and reflections when shooting through glass.

with a cpl filter(polarizing filter) :

Photo@ Brandon Nesbitt

Variable ND +CPL Filter (2 in 1)

This filter is designed for convenient use, combined with the function of the ND filter and the CPL filter together. If you don't want to take 2 filters when out you can use an integrated CPL + nd filter.

The K&F Variable ND filters with 5 stops for options

This filter is designed for convenient use, combined with the function of the ND filter and the CPL filter together. If you don't want to take 2 filters when out you can use an integrated CPL + nd filter.

Photo @ Armando Vernaglia Jr                CPL+ND Filter 

All in all

While both filters are quite similar and occasionally confused with each other, it must be noted that the ND filter is better at blocking light and does change the color of the image and the polarizing filter can enhance the colors and contrast within an image.

Do you use polarizers or neutral density filters with frequency in your photography? Share your thoughts in the comments below.