You can get low pass, high pass, band pass, and notch filters.
A wah is a band pass filter (I think, sometimes I get the names mixed up) that moved up and down the range and you move the pedal.
If the filter is controlled by an envelope follower, it's as though your picking dynamics are controlling the foot pedal.
You can get sequenced wahs as well, and autowahs which just rock back and forth at a set rate.
The are other types of filters, but I wouldn't really know about them in pedal form.