Correct, there are many kinds and then they can be used upside down, rotated and stacked also many strengths so gets confusing for many people sometimes because of all the choices.
Set the record straight:
There are two options for mounting ND filters to your lens:
1.) Buy 1 or more glass filters to screw onto the end of your lens
2.) Buy a filter system and a collection of ND filters to slot into it
Additionally there are 4 types of ND filters available:
