Cardiac pacemaker cells have beta-adrenergic receptors on their surface that bind norepinephrine and epinephrine. When norepinephrine, epinephrine and drugs like isoproteronol bind to beta-adrenergic receptors then there is an increase in the level of cyclic AMP (cAMP) inside pacemaker cells. Cyclic AMP activates funny channels and speeds the heart rate.
Other drugs like propranolol are "beta blockers"; they bind to beta-adrenergic receptors but prevent receptor activation.
β-adrenergic; "beta" is the name of the Greek letter "β".