One of the primary pieces of equipment in any backpacker's gear closet is the backpack.  There is not enough scrutiny that can go into the purchase of a backpack.  Besides often being expensive, the backpack has a great impact on how comfortable you are during your hike.  

The essential purpose of the backpack is to provide you somewhere to put a bunch of equipment and to place the weight of that equipment where  your body can most effectively handle it, the hips.

Backpacks come in two main flavors, depending on the construction of the frame, internal and external.  As you might guess, an internal frame is inside the backpack, where an external frame is outside.  Pretty easy so far, right?

Internal Frame Backpacks

The internal frame backpack has a narrow profile that integrates the framework inside the backpack, behind the shoulder harness.  The framework in most cases is a set of bars or, "stays."  These are usually aluminum, but they can be made from a number of materials and are removable so that they can be bent to allow the backpack to conform to your torso better.  Some manufacturers have moved away from using stays and instead use a "frame sheet."  This is a solid piece of rigid plastic that runs the length and width of the backpack and allows for better distribution of the load against your back.  Though the features and design of internal frame backpacks can vary greatly from model to model and from manufacturer to manufacturer, there are certain features that are common to all of the internal frame packs.

Comfort - Internal frame backpacks can be made to hug your torso much better than external frame packs.  This makes them more comfortable to wear and reduces "hot spots" where the backpack causes pain and discomfort.  Most of the internal frame backpacks out today also have complex suspension systems that can be used to tweak the fit to your particular needs.
Flexibility - The design of an internal frame pack is stiff, but not rigid.  Internal frame packs will move with your body better than external frame packs.  The tall and narrow design of these packs also allows your arms to move more freely, accounting for the internal frame's popularity with climbers.
Balance - Because the pack hugs your body, it brings the load in closer to your center of gravity where it is more stable.  This allows for more comfort on longer walks, better balance overall, and better control in situations where you might need it, such as a stream crossing or a narrow trail.

In spite of all the swell things that internal backpacks have going for them, nothing is perfect.  There are a few downsides to them as well.

Hot - This is a problem, especially in Georgia.  Because the pack hugs your body so tightly, there's little to no room for ventilation.  Consequently, you sweat.  Alot.
Poor Organization - Often, internal frame packs have one huge pocket for everything.  The "rest stop yard sale" is a sure sign that someone needed something out of the bottom of their internal frame pack.  Internal frame packs usually have fewer straps than the external packs; this makes for a more challenging time when you want to strap something to the outside of the pack.
Money - Internal frame packs are almost always more expensive than their external framed counterparts

External Frame Backpacks

External frame backpacks have been around longer than the internals, and as the name suggests, they consist of an external frame that the bag attaches to.  The frame is usually made of aluminum tubing, but some manufacturers have used plastic and even Kevlar as a frame material.  With an external frame pack, you can usually carry heavier loads because the load is situated more squarely on your hips.  Even though these packs are an old-timer compared to the new internals, they have some distinct advantages.

Cooler - Welcome to Georgia.  Every little bit helps.  Need I say more?  Seriously, the external frame pack sits off of your back and allows for more ventilation, keeping you cooler.
Organization - As opposed to the infamous uni-pocket of the internal frame, the external frame pack has many pockets.  This makes them easier to pack and easier to organize.
Money - They cost less than an internal frame pack.

Just as every silver lining has a cloud, so to do the external frame backpacks.  The downsides to these types of packs are:

No Tap Dancing - Because the external frame pack sits further off of your back, the load is farther from your center of gravity.  This makes for poor stability in any terrain where you might have to do some clever maneuvering.

Not as Adjustable - the suspension system on an external frame pack is usually less sophisticated than that of an internal frame pack, and consequently less adjustable.  Make sure you have a good fit at the store.