Lens filters are used to control/attenuate parts of the spectrum (colours of light) before they enter the lens/camera.  This can be in the UV (low end), visible (mid range) or NIR (high end) of CCD/CMOS camera sensitivity.

There are an almost infinite number of different filters.  They are usually described as low-pass, high-pass or band-pass.  The full description of a filter specifies a particular wavelength of light (the cut-off point or band centre) followed by low-pass, high-pass or band-pass.

The cut-off or centre wavelength is never absolute.  All filters have what is called a ‘roll-off’ characteristic that is usually available graphically.  This describes how accurately the filter is eliminating or passing unwanted light.  Generally, the steeper the roll-off, the higher the quality of the filter.