It can be a number of problems, some of them are simple and easy to fix.
Here's a few :
1. My first guess would be that your car is running on 3 cylinders (one of
the 4 stopped working)
Possible cause : loose or broken ignition wire, bad spark plug, cracked
distributor cap. If you look under the hood, you may hear or see some
sparks, which indicate the above.
Other possible cause : bad injector, broken wire or bad connection to the
injector. Sometimes a little wiggle on the wires and connectors give it
away, you can also use a small test light (the one that plugs into the
place of the injector) If you're not familiar with working on running engine,
don't try these at home.
2. Vacuum leaks can be a problem too, check your vacuum hoses.
3. Your engine can get really shaky, if one of the engine mounts is broken.
4. Even a bad temperature sensor can cause trouble, if it sends "cold"
messages to the computer, which results in a rich mixture.
5. The worst case is when the engine has some internal problem,
(rings, valves, etc.) You need a pressure gauge to find it out, (he one
that screws into the place of spark plugs.)
There are more, but this message is getting too long already...
Les