First a note about safety
Car batteries contain sulfuric acid. During the charging cycle, they generate hydrogen, the stuff that made the Hindenberg fall down and go boom. If you create sparks next to the battery, you can ignite the hydrogen, causing an explosion. Normally, your face and at least one hand is over the battery when this occurs. I've seen it happen IRL to my drunken stepfather, and it was not a pretty sight.
Most folks don't have the spare cash to go out and buy a new car battery when it starts getting weak. It takes a few jump starts before they're motivated enough to part with the cash. Another scenario is when you accidently leave the lights on when you go somewhere, only to return to a car that won't start.
Jump starting a car can be done in a safe manner, once you know the procedures. First off, inspect the jump start cables before using them. Normally they've been sitting in a hot trunk with a bunch of trash rolling around, and you want to make sure they are not damaged. Look for broken clamps and missing insulation chunks. Don't use them if they're damaged, you can get injured or damage one of the cars.
Park both cars close enough for the cables to reach battery-to-battery and open the hoods (bonnets for the non-Yanks). Have the other person hold a set of red and black clamps, and make sure you tell them to keep them separate. Don't hold them both in one hand, as they can arc across and cook some fingers.
Place your red clamp on your positive terminal of your battery. Have the other person connect the red clamp to their positive terminal. Next, have them connect their black clamp to their negative terminal. You will NOT connect yours to your negative terminal, however. This almost always causes arcs and sparks, which you don't want to happen (remember the Hindenberg).
Instead, connect your black clamp to the frame of your car. The best spot I've found is the hood locking mechanism. It will spark a bit, but it's not near the battery. Now you're ready to prepare a fast charge.
Make sure you turn off as much of your electrical equipment as you can, especially headlights and stereos. Rev up your engine a bit for a minute. This will help give the second battery a fast chunk of charge. Now, have the other person attempt to start their car. You'll notice a drag on your engine as your alternator supplies their car with the juice. If their car does not start, wait another few minutes, then retry. It should start up by the third attempt.
To remove the cables, first remove the one on your car frame. Then remove the others, making sure not to short the clamps together.