| In the early days of flying, runways didn't exist: you had airfields, which were essentially fields with airplanes in them. Nowadays, runways can look like this:
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| | | | | 3,000 ft
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| | | | 2,500 ft
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| || | || | 2,000 ft
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| || || | 1,500 ft
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==== | |_| | |_| | 1,000 ft
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| ||| | ||| | 500 ft
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| 3 6 |
| L |
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ooooooooooooooo
"Holy crap!" you're saying. "What are all those markings for?" Well, we'll go through them one at a time.
The first set of stripes, at the end of the runway, tell the pilot where the threshold, or the usable end of the runway, is.
Next, you have a number. The number tells you what direction the runway is pointing in, in tens of degrees. 36 means north, 9 means east, 18 means south, and 27 means west. If there's more than one runway pointing in the same direction, then the runway will also be marked "L" for "left," "R" for "right," or, when needed, "C" for "center." There are rarely more than two parallel runways: Dallas-Fort Worth holds the current record with six north-south runways, and accommodates them by numbering three of them "35" instead of "36."
The first set of three stripes marks the 500-foot point. This is where pilots are supposed to aim when they're landing. The boxes mark the 1,000-foot point, the double stripes mark the 1,500 and 2,000-foot point, and the single stripes mark the 2,500 and 3,000-foot point. These markings are mostly there so that pilots will know whether their plane is going to be able to stop on the runway or not.
Lights
If you thought that the markings were complicated, the lights will blow your mind, so we'll start with the basics. All lighted runways have, at the bare minimum, rows of white lights going down both sides. Most also have lights going down the center, starting in white, then alternating red and white for the last 2,000 feet, then red for the last 1,000 feet.
At each end of the runway, there are green lights pointed away from the runway, and red lights pointed toward it. The green lights are what you look for when you land, and the red lights are what you try to avoid when you're getting off the ground.
Many runways have two rows of red and white lights to the left of the runway. This is called the VASI, or visual approach slope indicator. The lights tell a pilot how high they are as they come in: if both lights are red, it means that the pilot is too low, and if both lights are white, it means that the pilot is too high. When the plane is at the right altitude for approach, the top light is red, and the bottom light is white (or, in the mnemonic of pilot instructors, "Red over white, all right").
The most major runways also have arrays of lights in the grass at each end of the runway, sometimes thousands of feet long. These are known as "alignment indicators," and come in many varieties: the most common are MALSR, MALSF, and ALSF.
Vital statistics
Most runways at international airports are one to two miles long. Widebody aircraft usually need at least 8,000 feet of runway, while smaller jets can get by with 6,000 and propeller planes can easily survive on 4,000.
The longest runway in the world is a strip of lakebed seven miles long, used by test pilots at Edwards Air Force Base in California. The longest in scheduled use, 13R/31L at John F. Kennedy International Airport in New York City, is a "mere" 13,000 feet long.
Runways are usually paved with concrete or asphalt, although they use much more material than highways since a fully loaded Boeing 747 can weigh up to 400 tons. In fact, runways are so strong that the French military had to develop a special bomb, the Durandal, to effectively destroy them. | 
