Neutral Density filter VS polariser filter

A bit into photography these days.

Here is the two main filter photographer uses and their difference

Neutral density

http://singhray.blogspot.com/2006/10/from-archive-using-graduated-neutral.html

Polariser

http://www.luminous-landscape.com/tutorials/polarizers.shtml

*main usage is for reflection and glare

*use polarise during the day , take it out at night because...

*polariser reduce F stop by 1.5