Filter and modulation pedals are an important group of effects that can change a guitarist's sound. Many of these are what you might typically think of when you hear the word "effects": phaser, flanger, chorus, etc. They change the source sound by adding another signal, such as an oscillator. Modulation effects include tremolo, chorus, vibrato, flange, phaser, and rotary effects.
Filter effects work by filtering out certain frequencies from the signal. For guitar pedals, perhaps the best known filter effect is the wah-wah or wah pedal. The envelope filter or auto wah is a variation of this, and their are other exotic filters like the Talk Box.