What is the difference between a chin up and a pull up?

Not a lot.

They are both compound exercises that work the upper body. Basically, the difference lies in the grip. A chin up uses a narrow grip with your palms facing towards you. A pull up is typically a wider grip with palms facing away from you. So which is best?

They both have their advantages. They work similar muscle groups, just in slightly different ways. You should use both varieties throughout the year.

If all you want is to do as many repetitions as you can, then by far the strongest grip is neither. The ‘hammer’ or neutral grip is in fact the strongest. This is where the palms are facing each other – usually about 15cm apart.

Do one pull/chin variety until your body starts to adapt, then change. Once you get used to, or sick of, all three then you can start changing other variables: tempo of the lift, angle, width of grip, repetitions, sets, total volume etc.