As was mentioned, the waiting areas are all different and are not available at all attractions.
Most are a roped off area of the regular waiting area, as shown in this picture montage.
Here is another - this is Monsters, Inc. Laugh Floor
Still waiting area is to the right picture. By the time that the show begins and doors open, the area to the left picture will be filled with people.
For this type of waiting area, you will know when to go in because the doors will open. Most of these are for shows and the main purpose of it is to separate off guests who will need to go to certain area (such as wheelchair accessible places in the show), guests who need a few extra seconds to get settled or need to wait in an area where they will not be packed in with other guests close on all sides.
Most are like these pictures, fairly narrow and with a wall on one side.
For these types, the doors will open or a Cast Member will come to get you.
For others - primarily rides, you will be waiting in a line and a cast member will come in time to go in or board. Some of them are a parallel line to the regular attraction line. Some are in a separate area. For most of those, guests with mobility devices have no choice but to wait in line, because the regular line is not accessible.
Guests will all wait in line or in the waiting area and be taken one party at a time to board.
In these situations, if you don't need the accessible line, your wait might be shorter going into the regular line. In the accessible line, you will need to wait for other people ahead of you with mobility devices. Depending on the number waiting, your wait may be significantly longer than if you had gone to the regular life.
So, if you don't need the accessibility features, look at the regular standby line before entering and see if that would be a better option for you.