There are basically three types of backpacks: Internal frame, external frame, and frameless.
Frameless packs are a specialty pack designed for ultra-light hikers/ rock climbers. Usually the user fashions a frame from a sleeping pad stuffed into the back of the pack bag.
External frames have the frame on the outside. Internal frames on the inside. Go figure.
Many people believe external frames to be more comfortable with a large load. They are definitely cooler, since there is an airflow between the back and the pack. They are slightly more unstable on rough trails.
Internal frames are more popular at this point in time. They tend to be lighter than external frame packs. They are more stable off-trail.
As regards women, internal frame manufacturers often offer internal frame packs for the woman’s body: narrower shoulders and wider hips. This is not to say that women always buy women’s packs: men’s packs often fit just as well, or can be adjusted to fit just as well.
The main item of interest regarding backpacks is how they are loaded. Most literature recommends that heavy items be loaded high in the pack. Women’s center of gravity is lower, so we usually find more comfort if the heavier objects are loaded closer to the small of the back.
Also be sure the pack fits. The easiest way to accomplish this is to go to a store which provides sandbags or other forms of weights and get the pack fitted. Failing this, bring a 20 or 30 pound bag of pet food and use that.
Generally the shoulder straps should just graze the shoulders, the load lifters (which most packs nowadays do boast) a few inches above the shoulder and about a 45 degree angle from the shoulders. The most common mistake I see is a pack with the load lifters BELOW the shoulders. At this juncture, they cannot in fact lift the load off the shoulders.
The waist belt should fit with the buckle at the navel. Some persons prefer the belt to be a few inches lower, but the buckle at the belly button means most of the load is balanced on the hip bones, which is where most of us balance a large load when we are saddled with a large load. The waist belt should be tight enough that most of the weight rests on the hips and legs. The shoulder straps should be loose enough that one’s hand should be able to slide between the shoulder strap and the chest.
If you do get a pack fitted, you are fitted FOR THAT PACK! Which is to say, if a North Face medium pack feels good, it is not recommended to then go onto Ebay and purchase a medium Mountainsmith. They probably don’t fit exactly the same. Figuring out that a medium fits well, and then going onto Ebay and purchasing an extra large pack makes no sense whatsoever, and yet I meet people all the time who do exactly that. Just because a pack is a deal, doesn’t mean it is the right pack for you.
The more "extras" a pack has, the more it will weigh. A top section which detaches to make a day pack is handy. Too many loops or buckles or daisy chains to hang stuff off the pack adds weight. If there are items to attach to the pack, such as sleeping bags or tents, spring for a pair of buckled straps. Tieing things on with a rope almost never works. A hiker jaunting down the trail with a loose bag dangling off the pack and bouncing on the butt is obviously a newbie.
Items which dangle, like extra shoes or a water bottle, will throw the hiker off balance. Light shoes are okay. Please don't fasten said items with a climbing carabiner which weighs more than the item. There are tiny biners sold for this purpose, or light velcro straps.