‘Cask strength’ or ‘barrel proof’ is the strength of whisky while its maturing in the cask. As we all know that when Whisky is taken out of Cask, its diluted with Water, typically spring water. The dilution is not necessarily done to decrease the strength but also to bring out the flavor.
Bottled Whiskey is usually at 40% Alcohol-By-Volume so you can imagine that the strength of Alcohol while its in the Cask is more than that. Its not very common to find bottles which are directly from Cask but sometime you do get one, for a very niche segment. But this is not very common, for variety of reasons. Probably one of the reasons is around the laws where you would need to mention the exact % of Alcohol and you would not want to print a new set of labels for every cask.
Also as the Whiskey ages in a Cask, its strength decreases and it may make more sense to not dilute it further if the strength has already gone down.
I could find this link – http://www.maltwhiskydistilleries.com/Distilleries/CaskStrengthEditions/tabid/99/Default.aspx, which has a mention of some of the popular ‘Cask Strength’ Whiskeys which have been released in market. By the way, I do not how much aged the above link is.
Enjoy your drink and have a good day.