Sparky.

I have attached a couple of images. The bottom has wires, plugs, and coils, and the top image is a coil pack. Coils and coil packs are basically the same things; the main difference is that coil packs are all together, whereas coils are separated. The coils/packs are basically the brains of the spark system that tell which plug when to spark. They are number 1-4 or 6 or 8, depending on how many cylinders you have. These can cause a number of issues either your car won't start, or its starts and shakes like the whole car is going to fall apart, or it will just idle a little rough. Luckily no matter what the case, it will generally throw a code that will light up the check engine light. If you do not own a code reader, any Autozone or O'Reilly will run it. Which will tell you which cylinder is misfiring