Smoke can come from the front or back of the car, and it's not good in either case. But the tailpipe will send-up colored smoke in attempt to tell you what the problem is. Here is your secret decoder ring:
Blue smoke: Oil is escaping from its intended passageways within the engine and is being burned along with fuel. Of course, you could always keep adding engine oil to the crankcase to prevent it from being all burned up (and risking serious engine damage), but the smart thing to do would be to take the car in to have any worn or damaged seals repaired [source: Torbjornsen].
White smoke: Water condensation or antifreeze has mixed with the fuel supply. Again, adding coolant or antifreeze to your car's cooling system will keep your car from overheating for as long as you remember to keep feeding the reservoir, but the wise move is to have it checked out as soon as possible.
If the smoke is coming from under your hood, that probably means you ignored white smoke coming out of the tailpipe, and now your engine is overheating. Or maybe you completely forgot to add coolant at all, and the engine overheated. Not that anyone would ever do that. That's ridiculous.