A PAC is a premature contraction coming from a piece of tissue in the atria. 

A PVC is a premature contraction coming from a piece of tissue in the ventricle. 

A small percentage of people, about 1 in 1000, have extra conducting tissue from the top to the bottom chambers.  This extra conducting tissue is termed an “accessory pathway.”  The majority of the time the accessory pathway does not conduct electricity. 

WPW is an abnormality in which the accessory pathway can conduct electricity forward from the atria into the ventricles even in the absence of SVT.  This means that it can be diagnosed on a routine ECG.