Beta blockers are "beta-adrenergic blocking agents," which reduce the ability for epinephrine and nor-epinephrine to attach to their corresponding receptors in the sympathetic nervous system. More simply, they slow the stress-related, fight-or-flight response produced by adrenaline.
Previously, research indicated that beta blockers have the potential to worsen congestive heart failure, but in the 1990s research indicated that beta blockers were effective at reducing morbidity and mortality in the condition. The beta blockers bisoprolol, carvedilol and sustained-release metoprolol reduce heart rate, while also increasing the oxygen carrying capacity of blood. Studies demonstrated that they reduce mortality over a 13-month period by 4.5%
The majority of this information was obtained on Wikipedia (http://en.wikipedia.org/wiki/Betablocker#Congestiveheart_failure).
There is also further information on specific beta blocking medications on FoundHealth:
Bisoprolol - http://www.foundhealth.com/bisoprolol/overview
Carvedilol - http://www.foundhealth.com/carvedilol/overview
and Metoprolol - http://www.foundhealth.com/metoprolol/overview
Hope this answers your question. Feel free to ask any follow-up questions as necessary!