Packs typically appear in one of three layout configurations:
Top Loading
Front/Panel Loading
Compartmented
and then there are the pockets

On EF packs, compartments typically feature a main compartment on the top, a smaller compartment on the bottom, and numerous outside pockets. The zippers typically run straight across the top of the compartment or pocket. Some design variations may incorporate half-moon zippers, or vertical zippers on the pockets to make more of the contents visible during access. Straight-across zippers are less preferred, since the zipper opening can limit access to equipment inside. IF packs typically feature a large main compartment, a sleeping bag compartment, and at least one outside pocket. The large main compartment may often have internal compartments or separators.

Top Loading
Top loading packs in the strictest definition of the term are packs that can only be loaded from the top. "Straight through" top loading packs are basically one large compartment . Straight-through top loading designs are generally less expensive - they're simpler designs using less material. Some may have internal compartments as well. It can be a challenge positioning gear in an optimal "load" configuration with a straight-through design, but it can be done. Getting your sleeping bag out at night usually means unloading the entire pack in the tent. If you're not rich but still want a good quality pack for a reasonable price, a straight through top loading design may be a consideration.

It is also worth noting that some top loading designs also employ an etendable top, which can increase the carrying capacity of the pack. In some cases, this can amount to an extra 20% of space. However regularly relying on extensions for routine use is not preferred, since the "carry" is unlikely to be as good as a larger pack.

Of course, there are thousands of pack design variations. There's no way for us to categorize each one. Leave it to say that you can find these designs in your local outdoor shop, or countless combinations and variations. It's up to you to decide what you like, what you can afford, and what works best with the gear you intend to carry.

Front/Panel Loading
A medium size panel loading pack - the Kelty Redwing. Some backpackers shy away from panel loading designs in full-size packs because of a perceived fear of zipper failures.

Front, or "panel" loading backpacks work well for school packs. The front loader typically features one large compartment with a zipper around the perimeter. The advantage of this design is that the pack can be opened to expose all the equipment or books contained inside. This can make locating a specific item of gear very easy. The downside is that if you have some highly compressed clothing inside the pack, it will uncompress as soon as you unzip the access panel. I have often heard off-hand comments about the possibility of zipper "blow-outs". Granted, with all the contents of the pack pressed against the zipper, this is a logical concern. But I have yet to see it happen, and the zippers used on these packs are typically huge. Manufacturers also typically address this concern by adding compression straps across the back of the pack to relieve zipper pressure.

Internal Compartments/Separators
You're more likely to find internal compartments or separators on IF packs than EF packs, but they can appear on both designs. Internal compartments and separators allow you to compartmentalize your equipment and provide ideal positioning of heavy equipment. The higher and closer to the back you position heavy equipment (such as stoves and extra water), the better you can balance the pack while walking. Compartments and separators can also make it easier to find your gear while you're on the trail.

Some pack designs may have zippers, buckles, or Velcro on the compartment separators, which allow you to adjust the size or number of internal compartments depending on what you're carrying inside. Others might have holes in the corners that will allow you to slide long items such as tent poles inside the pack. There are lots of variations. Decide what you think will work for you.

Pockets
Pockets allow you to compartmentalize and locate your equipment quicker. You'll generally find more pockets on the outside of EF packs versus IF packs. EF packs usually have three to five outside pockets, whereas IF packs may only have one or two (however, these pockets are large). Many IF pack manufacturers offer accessory pockets that can be purchased separately and attached to the pack if you absolutely feel you need them. While more pockets on the outside can offer quicker access to equipment, many IF manufacturers also include a large zipper down the side or back of the pack that can allow you to access the main compartments. Sometimes it's easier to have clothing in the main compartment rather than jammed into a small outside pocket. Clothing is the most frequently accessed item of equipment when you're on the trail aside from water.

On the subject of water, some manufacturers have started to included special mesh "water pockets" that are open on the top. You can slide a Nalgene water container into one of these pockets without taking the pack off. Integral water bladder incorporated into the top pocket of the pack is becoming very common. Along tube runs from the bladder to the side of the pack where you can quickly "catch a sip" without removing the pack, or even reaching around for a water bottle. (The downside of this design is that you never know how much water you've consumed unless you take the pack off and check the level in the bladder. It's up to you to decide if you think this is a worthwhile feature.)Water bladders and drinking tubes can be purched seperate and can be added to most packs afterwards. Others may prefer to carry their water on the side of the waist belt using a hip or waist belt mounted bottle carrier. The carriers are inexpensive, easily detachable, and put the weight farther forward on your body instead of adding more to to the load being carried on your back.