They are all geared for different purposes. Meguiar's is actually geared towards professional use, and then (for the most part) the consumer line spawns out of that. The NXT line is the exception, which started with NXT wax, and then spawned into M21 in the professional line.
For the most part, you have 5 different lines: Consumer, Professional (Mirror Glaze), Professional #2 (Detailer); Motorcycle, Boat/RV.
In the Consumer, you will find their Deep Crystal, NXT, Hot Rims, and Gold Class line for the most part (ColorX, ScratchX, PlastX, Cleaner Wax, Natural Shine, Quick Interior Detailer, Ultimate Quick Detailer, Quick Wheel Detailer, and Quik Out are also part of the consumer line). These are products that you will usually find in the retail stores, and their made for ease of use for the consumer. Deep Crystal Polish is a good example of this; it is essentially M7 but just made easier to use.
The Mirror Glaze line is the professional line in the Meguiar's lineup where all the top quality products can be found. These are geared towards professionals and body shops, and is the oldest line of products they have offered since the beginning over 100 years ago. Many of these products are made to be used with a Rotary whereas the Consumer line is not. The focus for this line is the quality and performance, and rotary where applicable. Also, many of these products can be painted over, hense the focus on the body shop professionals.
Detailer line is meant for detailers where volume is intended. These products all have a pleasant smell to them and sold in volume for detail shops and the smell to make them pleasant to use since you will be using them all day.
The other two are pretty self explanatory. Hope that helped.