Bottling is the last step before putting the whisky on the market.
Unlike wine, whisky does not mature anymore in the bottle. So a 12 years old whisky stays a 12 years old even 12 years later, and does not become a 24 years old one....
When bottling, some residues are left in the whisky. The effect of this is that whisky looks "cloudy", and this is not always appreciated by the consumer. That's why distilleries found out the "chill filtering", which removes all this residues. The problem with chill filtering is that it also removes parts of the fragrances and of the taste.
With the current revival of single malt, more and more bottlers (in dependant or official) bottle their whiskies without chill filtering. And this makes single malt lovers very happy.
During bottling, the alcohol percentage is reduced. This is the other operation where the quality of water has a great influence on the taste of whisky. The minimum percentage of alcohol for whisky is 40%. Most of the bottles are marketed at this percentage, because the excise rights are calculated on the alcohol proportion in the bottle. The excise rights are particularly high in Great Britain, but in other countries they are lower. That's why on the international market, whiskies are frequently bottled at 43%.
For some technical reasons, the ideal percentage for bottling without chill filtering seems to be 46%. Most of the non chill filtered whiskies are marketed at 46%.
Often whisky is not diluted when bottled. That's called cask strength bottling.
Generally, the casks are mixed before bottling, to get a more standardized product, just like great wines. When the whisky comes from just one cask, it is called "single cask".
Most of the distilleries do not bottle their own whiskies, but let this happen at specialized plants.
Exceptions among others are Glenfiddich, Springbank, Bruichladdich and Loch Lomond. Even if they do not bottle themselves, the responsibility of the bottling stays from the distillery. This is called "official bottling". This operation happens often in the suburbs of Edinburgh where several bottling plants are installed, belonging to distilleries (like Glenmorangie in Broxburn) or to independent bottlers, like Ian McLeod in the same town.
A very interesting phenomenon in whisky world is the work done by independent bottlers. Unlike bottling plants who work on behalf of distilleries, the independent bottlers buy casks at one or more distilleries, choose the type of cask, and let it mature in own warehouses or in the distillery warehouse. The independent bottler decides when the whisky will be ready for selling. These bottlings are marketed under the name of the bottler, and sometimes the name of the distillery does even not appear on the bottle -rarely-. Some of these companies are Signatory, Ian MacLeod, Douglas Laing (Provenance and Old Malt Cask), Cadenhead, etc...