The term reverb refers to the reflections that naturally occur when a sound interacts with a surface. When a sound occurs in a closed environment such as a room there is a natural presence create in that room in the form of some surface reflecting back to the sound in various ways.
This effect can be recreated by reproducing a large number of reflections with slightly different timings, stereo field and phase differences as well as other factors in rapid succession. Within Logic there are 4 reverb units that allow for different method of recreation of reverb and each have their own character of sound and functionality.
Common controls are found across multiple type of reverb plugins and where some may use different terminology they will serve the same purpose but be algorithmically different in how that effect is achieved.
Predelay - Is the time between the initial sound being created and the arrival of the first reflection back.
Attack - Is the length of time the returning relations take to reach maximum volume
Density / Size / Reflection - Refers to the number of reflections that return based upon the original sound. This can be referring to the number of surfaces in a room for example whereby each surface will produce a reflection.
Dry - The amount of dry signal which is the original sound that come though the reverb unit, This will only usually be used when the reverb is placed onto the source.
Wet - How much of the reverb signal leaves the reverb, usually in terms of gain but represented as a percentage. This is the most common way to work with a reverb by only outputting the wet signal and sending multiple sound to a single unit.