Matt gives an OK description, but it is missing some important points, such as attachment order, so please consider these instructions:

• 1) Park the good car so that its battery is as close as possible to the dead car's battery, and turn off the ignition.
• 2) Expose the battery terminals and attach one end of one cable to the + terminal of the good car.
• 3) Attach the other end of that cable to the + terminal of the dead car.
• 4) Attach one end of the other cable to the - terminal of the good car.
• 5) Attach the last remaining clamp to exposed metal of the car somewhere other than the dead battery. This is to avoid potential explosions from sparks and leaking batteries. Anyway, I remember the full order as "good plus, bad plus, good minus, bad car."
• 6) Start the good car, leave it in park, and gently feed some gas to rev the engine for a half a minute or so.
• 7) Have the other person start their car. If it doesn't start, you may need to feed fuel to the good car's engine for several minutes before trying again.
• 8) When the dead car starts, remove the cables in the reverse order that you clamped them (bad car, good minus, bad plus, good plus).
• 9) Don't turn off the bad car until you are in a place where it's OK for it to die again.

(Edit: Gave the list bullets to be easier to read.)