What is Swing?
"Swing" is a personal interpretation of rhythm based mostly on 'feeling'. When music is not played swing, it is referred to as straight. Some musicians are said to have a harder swing than others, meaning that their interpretations are farther from a straight version of a piece than others.
To illustrate what a swing rhythm may sound like, try to play a short series of 8th notes straight, but play the last note slightly too late. There you go, you are already 'swinging'!
Note: EarMaster's Jazz course includes hundreds of rhythm live tapping exercises with Swing.