On a simple level, saturation is a fancy word for clipping: Once the input voltage goes higher (or lower) than a circuit can handle, it is instead held at that limit. However, saturation usually implies a more rounded, shaped approach to that clipping limit, resulting in a more pleasing (or at least less annoying) form of distortion. Tubes circuits are often associated with this soft clipping behavior, although it can be emulated in other circuits or even digital signal processing. Different devices may be sought out for specific sonic character of the way they.
