When it comes to ultralight backpacks, you can find basically two groups of packs: frameless & internal body packs. External frames are a different animal altogether and, because I have no experience with them, I cannot give an objective opinion of them. Yet in the world of ultralight hiking & backpacking, external packs don't generally have a place. This article reveals some basic comparisons with frameless & inside frame packs but, by no means, is this a comprehensive report.
First off, what are the main differences of frameless & inside body packs? Frameless packs don't have what are called stays, or a basic frame outline which supports the structure. These are usually made from aluminum, fiberglass or carbon fiber. Strength is maintained with these lightweight materials. It's generally accepted that a pack at 3 pounds or less is considered ultralight, and some may even stretch that to 3.5-4 pounds. You will find many quality-built packs within the realms of these two types. Understanding what kind of hiker or backpacker you are will go a long way in determining which type of pack you should buy.
Frameless packs basically cannot stand on their own, and when empty, crumble like an empty duffel bag. They are made from thin nylon, strong & abrasion resistant. These packs have become incredibly popular over the last several years, and with the advent of newer materials, these packs just seem to keep getting lighter & stronger. I've kind of had a love-hate relationship with my Granite Gear Virga backpack. It's a frameless pack, and when full, can comfortably carry up to 30 lbs. I've had it stuffed loewe goya backpack to more than 35 lbs of gear, which isn't recommended by the manufacturer.
Loading the pack first thing in the morning, after just rolling up my tent & cold, maybe a bit cranky from a not-so-good night's sleep; more than once brought me near cursing. This is the most common downside of frameless packs, and many people don't consider it a downside, just a unique character trait. The idea behind loading a frameless pack is to take a rolled foam sleeping pad and place it long ways into the pack, loewe sale allowing the pad to unroll and therefore shore up the loewe store sides somewhat, for support.
Inner body backpacks can generally carry larger loads than frameless packs & have more support in the back & waistbelt areas of the pack. A number of manufacturers build good internal frame packs. These packs are commonly believed to hold packed gear closer to your body than external frame packs do, but are comparable with how they fit with newer frameless packs.
Both packs sit against the back, allowing for minimal ventilation in this area. Even with the newer, "breathable" pack rests on the market today, a person's back will become wet with sweat from wearing the pack. This is something that manufacturers have tried to mitigate with modern designs, but it's an inevitable point in wearing either of these type of packs.

Frameless Backpacks Vs Inner Body Backpacks