Cardio-selective beta-blockers primarily block $\beta_1$ receptors, which are mainly found in the heart, with less effect on $\beta_2$ receptors located in the lungs and peripheral vasculature. Atenolol, betaxolol, and esmolol are examples of cardio-selective beta-blockers. Timolol, on the other hand, is a non-selective beta-blocker, meaning it blocks both $\beta_1$ and $\beta_2$ receptors.