Ectopy. The heart beats by electrical conduction system through the cardiac muscles - sometimes it can skip a beat or beat irregularly - these are known as arrythmias. There are benign kinds (such as psvt, pvc) and more dangerous kinds such as atrial fibrillation. An ekg will give you the answer.
Extrasystoles. Usually it means you're having premature heart beats (Pac's, pvc, s) and the skip is actually a pause following the early beat. These premature beats can be caused by caffeine, alcohol, nicotine, anxiety, overactive thyroid or actual heart disease. It's best to get checked with an EKG or heart monitor to identify the heart rhythm and type of extrasystoles and then get other tests or cardiology consult.
Skips. What is usually happening is an extra beat which feels like a 'skip'. This is very common and doesn't necessarily have any evil import. If you have a lot of extrasystoles (skips), it is worthwhile seeing your dr. To get checked out to be sure there isn't an underlying problem.