Theres one coilpack per cylinder, they boost the electrical power to make the plug spark, which ignites the fuel etc..etc. Hope this is fairly accurate /ubbthreads/images/graemlins/crazy.gif.
When the coilpack fails your engine develops a misfire because you are only running on 3 cylinders ie it runs like a dog. You aren't advised to drive too far, even though the car will run, because it can feck up your catalytic convertor.
Mine went before Xmas and I drove 15 miles to my dealer, I assume this didn't damage the cat because they didn't say anything when I picked the car up.
Christian