"Chopping" is dissecting the song that you are sampling so that you "chop" out the part or parts of the song, be that the bassline, rhythm guitar part, drum break, or other music, you want to use in the beat. Looping is known as melodic or percussive sequence that repeats itself over a period of time, so basically a producer will make an even-number of bars of a beat (e.g., four bars or eight bars) repeat itself or "loop" of a full song length. This loop provides an accompaniment for an MC to rap over.