Not just great for your biceps, chin-ups are equally effective for overall upper-body strength. They also hit your grip, forearms, back and core.
With your palms facing towards you, grab a bar or rings at shoulder width or less (A).
Keep your core tight, and raise yourself using your biceps and back muscles, to a point where your chin is above the bar or bottom of the rings (B).
Hold for a second or two, then lower yourself until your arms are fully extended.
Think about pulling your elbows through your ribs to engage all of your back muscles.