Okay, let's cover the suspension basics
Pack suspension has seen impressive evolution over the last 100 years and is now a sophisticated set of design and engineering practices. If you need to distill it to one principal, it is the goal to evenly distribute pack weight onto the human body to limit fatigue and keep the center of gravity as close to the body as possible.
While framed backpacks go back hundreds of years, it wasn't until after WWII that pack makers begin to experiment with aircraft tubing to make lighter and more rigid frames. During this time, the first instances of attaching the frame to a waist belt appeared. This was a revolutionary idea as it brought about a new approach to loading weight onto the human body. For the first time, much the weight on one's back could be distributed to the hips. This allows people to carry more weight longer without prolonged compression of the spine.
This post will mainly discuss larger outdoor bags. Frames are not always better on small to mid-sized bags. Any daypack or small backpack is better off without the size and complexity of a waist belt and frame.
Getting the right fit
Before getting into all the details of suspension, it's important to note that all the bag structure in the world won't help if the pack length doesn't match your torso. When first fitting a bag, get the waist belt so it is sitting on the hips just above the pelvic bone. Next, adjust the height of the shoulder straps. If a bag is too long or too short, it should be immediately apparent. For minimalist hiking bags, the user often needs to find a bag that matches their torso length, but many higher-end bags will have an adjustable "yoke". This this a moveable shoulder strap panel that can be adjusted to match the user.
Main approaches to suspension
Frame sheet (plastic):
This is one of the most common approaches to weight transfer. A plastic sheet, usually polyethylene or polypropylene, serves as the rigid element to transfer weight to the waist belt. The benefits include a semi-flexible back panel that is sometimes even removable and relatively low cost. Plastic frame sheets are most common on packs that need to support 20 pounds or less and for this reason, are usually found on small to medium volume packs.
Frame sheet (wood/fiberglass or carbon fiber composite):
Larger packs that will take more weight (above 20 pounds) require a frame with greater rigidity. One way to do this is by adding a baton or stay (see below). Another approach is using fiberglass or even carbon fiber composite materials. These materials offer greater rigidity and often with less additional weight. Designing and manufacturing composite materials is much higher-cost and, for this reason, is usually only reserved for larger and higher performance packs.
Frames heet+Stays or Batons:
This combination is one of the most popular frames and is sort of a sweet spot between ease of manufacture and performance. Simply put, a baton or stay (which is just a stick) is attached to the frame sheet. The stay takes the vertical loading, while the sheet helps distribute that rigidity across the entire back panel. Stays come in a number of materials including aluminum, fiberglass, carbon fiber and even steam-bent wood! The stays are often formed into an "S" curve to better match the the curve of the spine. Some stays, mostly aluminum ones, are able to be formed by the owner to fine-tune the curve to match one's back.
This frame style is becoming more and more popular in recent years. Perimeter frames are usually made of aluminum or steel rod or tubing and perform similar to a frame sheet+baton but with fewer parts. The benefit is a slightly more efficient structure with a little less weight. These frames have the added benefit of leaving the back panel open for rear access.
This is actually still a perimeter frame but is combined with a fabric or mesh. With these designs, the perimeter frame is bent into an arc shape that arches away from the spine creating an air gap between the bag and the user. The fabric mesh spans the frame and is held in tension . . . like a trampoline. This style bag is great for longer hikes in warm climates because you get the benefits of a structured pack that is only touching the body at the hips and shoulders, allowing for better ventilation.
One of the main ways to adjust the shoulder straps and the way the frame sits on the upper body is with load lifters. These are straps connect from the upper part of the shoulder straps to the tops of the frame, which needs to be above shoulder height. The resulting triangle shape allows for fine-tuning of weight distribution on the shoulders.
In recent years it's become fashionable to add load lifters to backpack straps for appearance. These are easy to spot because the shoulder strap and lifter strap connect to the bag in the same place and do absolutely nothing but add weight to your bag.