This post is in response to a direct request from SGT Danger, who has experienced a change of mission.
First, read some basic history on Afghanistan. You don't need to know a ton, but being familiar with the history there is a good idea, and Afghans are very impressed with someone who has taken the time, and had the respect, to learn about their history. Afghanistan has a long history and is a witness to many empires, most of which have run over Afghanistan like steamrollers. Afghanistan has been like the cartoon character who is run over by a car, struggles to his feet and has scarcely dusted himself off when he is run over again. And again. And again, ad nauseum. Afghans, particularly the Pashtuns, have been called xenophobic, and while they have some xenophobic tendencies, it is this role as the speed bump of history that has ingrained this.
In your research you will find that the Persians, Alexander the Great, Tamerlane, Genghis Khan, Kublai Khan, and more recently the British Empire and the Russians have all swept through Afghanistan. For some, this paints a picture of the indomitable Afghan. I tend to disagree, as the Afghans have indeed been conquered on numerous occasions. However, Afghanistan has never been the prize, more like a necessary bridge from where the conqueror was to where he wished to be. What the Afghans are, however, is survivors. The ominous name "graveyard of empires" is a misnomer. None of the great ancient empires were undone in Afghanistan, but Afghanistan was instead a way to measure the waxing and waning of these empires. They all swept through on their way to expansion, and then had to retract through Afghanistan again on their way back whence they came, leaving their genetic mark on the land. The Afghans, however, have survived. Afghans are not indomitable; they are consummate survivors, amazing in their flexibility and often playing foreigners off of each other and their domestic competitors.
More recently, the British and Russians have found great difficulty in Afghanistan, mostly through their own idiotic mistakes. These experiences in particular are held up as some sort of omen as to the fortunes of the NATO mission in Afghanistan. I caution you not to give much credence to such examples, for there are significant differences. No one through history has gone to Afghanistan for the sake of Afghanistan. What we are doing in Afghanistan is for their sake, but do not become confused; it is not because we are so selfless. It is because by doing the right thing in Afghanistan, we make ourselves safer. Do not buy in to any thoughts of whether or not they deserve our assistance. The question is in itself diversionary. We chose this mission eight years ago because it is our best interests. The Afghans need a lot of help. Theirs is a society that has been developmentally disabled by thirty years of warfare. They have forgotten how to govern even as well as they were ever governed. Forty years ago, Afghanistan was on its way towards modernization. Events since the deposition of the king in 1973 (the king died in August, 2007) have taken Afghanistan back until they are now ten minutes out of the stone age.
If you think of Afghanistan as an individual, this would be a person who has suffered repeated blows to the head and suffers from TBI and PTSD.
A basic understanding of this will assist you in your observations of Afghan behavior. Observations of behavior are critical; your best way to prepare for danger is to be able to recognize what normal looks like. It is only through learning what normal looks like that you will have any hope of recognizing what abnormal looks like. Being able to recognize abnormal behavior or circumstances will help you to stay alive and keep your Soldiers safe. At first, when you arrive, your "Spidey sense" will be alerting you constantly, overloading your mind and your emotions. Relax. Learn. In a short time (2-3 weeks) you will have seen much of Afghan behavior enough to know (mostly) what normal looks like.
Expect to see crushing poverty. Expect to see children who appear to be about four years old herding goats or sheep off by themselves in the middle of the day. Expect to see more Toyota Corollas than you ever thought were built. The general feeling has often been described as Biblical times blended with the Wild West with a touch of Mad Max.
Do not confuse illiteracy with stupidity. Afghans very often learn quickly by observation. They have a strong tradition of oral history. Be aware of why they are consummate fence-sitters, the ferocity of their lack of commitment born of a strong survival instinct. Understand that, often, what we see as corruption they see as the price of doing business.
Be slow to judge them by American standards. While the easy answer, it will only breed discontent in your own soul. There are many Afghans who are very glad that you are there. If you have close contact with them, you will quite likely be thanked by some for being there. There will be more on culture in further chapters.
The link to the history of Afghanistan above is to Wikipedia's good synopsis of Afghan history. It's not terribly long, and it provides links to any particular area you'd care to explore.
An excellent introduction to the modern history of Afghanistan and the development of the Taliban is National Geographic's "Inside the Taliban." This can be found in ten parts here (follow stu106 thread of ten parts on YouTube.) It can also be downloaded in full here.
There are other websites with more anthropological examinations of Afghanistan, like Registan and Ghosts of Alexander. Both are written by academics and offer insight that can be helpful. Joshua Foust, author of Registan, recently returned from Afghanistan.
Finally, if you get a chance to catch a screening of "At War," a documentary film by independent journalist Scott Kesterson, miss an entire night's sleep to do so if necessary. This film will give you a sense of what it's like on the ground. It has been known to make veterans of Afghanistan experience the same rush of combat they felt in country. It's that good.
Once you have completed the above ("At War" film optional based on availability,) you will have a passing knowledge of the land for which you are bound.