But of course, that isn’t what a filter started as! Before digital, a filter was simply a piece of glass that you put on your camera lens (and it still is, of course) to create an effect. At its simplest, a filter could be a UV or Skylight filter which had minimal affect on the image and is mainly used as protection between your expensive lens and the elements.
Using common analog filters
For B&W photography, color filters are quite common; whatever color filter you put on, those colors become lighter/brighter in the scene and their opposites become darker. So a yellow or orange filter will brighten skin tones, and a red filter will brighten red tones, while making blue skies almost black.
The neutral density filter allowed for a long, multi-second exposure
Applying digital filters
Now that we are mostly shooting digital, you can of course still use analog filters. In some cases it’s better to use analog (you can’t magically see through the surface of a lake using a computer filter to replace a polarizing filter, for example), and in some it’s better to use digital (a red glass filter for B&W has one, permanent setting — but doing it digitally give you infinite options with much more control).
Software like DxO FilmPack gives you incredible control over B&W conversions, allowing you to channel mix individual colors as black and white to brighten or darken specific tones.
Here’s a before and after of an image converted to black and white using the Kodak T-Max 100 film preset with no additional affects applied, compared to the same image with the yellow and green channels raised (to brighten foliage), and the blue channel reduced (to add contrast to the clouds).
In this screenshot of the Presets page in DxO PhotoLab, you see a lot of very different “looks”. These aren’t technically filters, but actual presets of settings that combine together into an easy to apply, well, filter!