Acoustic effects are generally deemed to be those guitar effects that are applied to or used in conjunction with an acoustic or electro acoustic guitar. They are usually a little more conservative and subtle than effects applied to an electric guitar as in most cases guitar players strive to retain the "natural" sound of the acoustic guitar within an electrified domain.
As with electric guitars, electro-acoustic models have pick-ups, transducers or built-in microphones to help to amplify the sound, therefore it is possible to directly apply guitar effects to enhance or modify the tone of the instrument.
Typically, the type of effects that are used on amplified acoustic or electro-acoustic guitars are EQ, chorus (to simulate a 12 string guitar) and a compressor/limiter effect which fattens up the sound, as well as ensuring that each tone or chord is of an equal and consistent volume level.