There are a variety of problems that can cause a car's idle to be rough. A rough idle is usually defined by the car idling below its normal speed, loping RPMs, a skipping sound or in a way that causes the car to shake. Idle problems can often be a clue to something more serious going on, so it is best for car owners to get them checked out.
The simplest reason for a rough idle is either a vacuum leak or an incorrect idle speed. A vacuum leak causes air to get into the system and disrupt the normal idling speed. Idle speed is a setting on most cars, and some cars require only a simple tool like a butter knife to correct this. However, the idle speed should not change randomly and another problem is likely to be lurking beneath the surface.
Dirty fuel injectors are another possibility for a rough idle. Fuel injectors do need to be periodically cleaned throughout their lifetime as fuel deposits build up and eventually interfere with the car getting proper fuel. Lack of fuel is similar to lack of air and the idle speed is affected. Other possibilities include spark plugs or the distributor cap and wires.