Neutral density filters?

From the you tube video below I learnt about a new filter called the neutral density filter. What i learnt that was although i know to make a landscape in perfect focus in the depth of fields, i knew that you had to set your camera to the biggest aperture your camera would go to like F22-25.This then allows you to have a longer shutter speed, before i knew the details of how to use a neutral density filter i used the technique above but over expose to get a longer shutter speed then darken the image down a lot in photoshop. I never knew there was another way of going about it until i learnt about the neutral density filter. From what i learnt from the you tube video that when you have your camera on F22-25 the neutral density filter allows you to have a much longer shutter speed compared to not using the filter, hence allowing more time to experiment with blurring(of water, sky's,trees blown by wind etc), filters and effects.