I am not an expert on car electronics and I will let someone else with more experience take a lead.
Essentially the concept of 1-Din or 2-Din depends on the number of units you are looking at. A Cassette player, a CD Player, or a tuner can each fit in into a 1-Din format. Most of the receiver units also come with a Class D amplifier that does not take too much space. If you want a HU that have 2 or more of the above individual units, you have to look at a 2-Din format.
Currently LCD screens have also become popular and something like a 7 inch screen will definitely take a 2-Din format.
If you want a powerful 4 channel amplifier with possible connection to a sub, you have to place this elsewhere in the car such as the boot.