Today the little ones and I went to Red Rock Canyon. I brought along some ND filters to show what they can do.
ND filters are basically like putting sunglasses on a camera. They darken the whole thing. There are a few reasons to want to do this. Wanting to use a large aperture to have a shallow depth of field, wanting a long or longer exposure time, making flowing water silky, removing people from a busy location. They come in different strengths. Just like CPL filters they can be costly and we can buy one set for our largest thread lens and then step rings for the rest of the lens. Ultimately we want a filter that darkens the frame evenly without adding a color cast.
There are also graduated ND filters which are used to darken a bright sky over a landscape so both can be seen. They come in both screw on filters and in a square filter with a holder. Just like the regular ones also come in different strengths.
As for removing people from scenes I have pictures of them removed but no before shots. If I go to a populated area where people are moving I will get it. Until then see where an ND might fit into your needs. Now go out and shoot.