Filters are thinner than washes, usually about twice as thin (that is, twice as much thinner is added). I'm talking about a "filter" here, not a "dot filter."There is much confusion about these terms. There are different types of washes ("pin washes" and "sludge washes") and different types of filters ("filter" versus "dot filter"). To add to the confusion, figure painters and artists use these same terms differently. Here is a description of each term as used by the modelling community.
Filter: A filter is a product you can buy from companies such as Mig Productions. They are generally very thin enamel-style paints meant to be applied to large areas to create a subtle change in color. You can also make your own filters with oil paints or enamels (or any kind of paint, really) by mixing 1 part paint to about 20 parts thinner.
Note: A figure painter would call this a "glaze." A watercolor painter would call this a "wash."
Dot Filter: A technique that is unique to the modelling world. You take several colors of oil paint and apply small dots to the surface of the model, usually straight from the tube. Then you take a brush and blend out the dots until you end up with a bunch of small areas of subtle color variation.
Wash: The wash is a product you buy in bottles from companies such as Vallejo or AK Interactive. You can make your own by mixing 1 part paint to 10 parts thinner.
Pin Wash: Short for pinpoint wash, but sometimes just called a "wash." You apply this wash by touching your brush to a recessed panel line or to the edge of a raised detail and let the paint flow by way of capillary action. Many prefer to aplly this over a gloss surface, but a matte surface works just fine.
Sludge Wash: I'm not sure what this is exactly, but I think you apply it over a large area and then wipe it off. Paint will remain in recesses and around raised details.