There are various types of distortion on the Korg A1, from a very light crunch to heavy overdrive. The Korg A1 was designed to be used with a guitar for both recordingand at live performances, so combining distortion with other effects can give your live performance a heavy, loud sound. Distortion may also be used in recording and mixing to create all kinds of sound combinations, including the retro 8-bit sound.
Reverb is short for reverberation, and generally speaking, describes the level of direct to reflected sound that runs through it. Reverb is used frequently in mixing to give recordings a bit more space, and to separate different instruments in the mix. The reverb effect will simulate the sound you run through the A1 in a variety of "spaces," from small wooden chambers to large concert halls. Using some light reverb in a live setting can also be useful in making your instrument sound a little bigger.
A chorus effect will take your original sound and give it a number of "voices," replications of that sound with slight variations in pitch and delays. With the Korg A1, you can select the amount of voices you would like, as well as your LFO, which controls the rate of the effect's delay. Chorus effects can be heard on many recordings to create subtle harmonies and are thought to be especially useful for piano, guitar and synthesizers.
Delay is a very simple effect, but important nonetheless. A delay effect will play your sound again, only slightly more delayed and softer than the first instance. An echo effect is a type of delay. Using the Korg A1, you can program complex delay patterns, in which the sound can be repeated several times, with different time variations. Most recordings and live performances will route their sound through small amounts of delay to spread it out, allowing it to envelop a whole room.