Afib- normally there is one place in the right upper chamber (atrium) that “controls” the heart rate- it fires an electrical charge that is then transmitted through the upper chambers,, causing them to contract and squeeze blood (the lub in lub dub of the heart beat). the signal then passes through a spot between the upper chambers and lower chambers, then goes through the electrical system of the ventricles, causing them to contract and squeeze blood- the dub in lub dub. Result is blood being squeezed through the heart and around the body to get oxygen and nourish the body organs.
In atrial fibrillation, the electrical system goes a bit haywire, and multiple places in the upper chambers fire off in a chaotic rhythm, causing the heart to beat in an irregular and less efficient manner. See Web MD or other online sites for pictures of this.
Tachycardia is just a term for a heart rate over 100 beats per minute, it can be normal, from exercising, or abnormal for a lot of reasons.
And PVC is Premature Ventricular Contractions, or the squeezing of the lower chambers that is not happening when it should in the heart beat cycle. PVCs are very common, not dangerous unless associated with a heart attack or some other conditions. Some people feel them (“palpitations”) and some don’t.
Hope this helps, and if anyone has corrections, fire away 🙂