Buttermilk is a natural dairy product, made from churning cream — something which has been done worldwide for countless centuries. Buttermilk appears in most cuisines from Russian oladi to Indian neer more, and buttermilk-fried chicken from the Deep South.
It's Irish dairy farms which are most closely associated with buttermilk though — using it in the national recipe for soda bread. The small, independent farms would save up their milk until they had enough to churn. So by the time that the fat had split into butter, leaving the buttermilk by-product behind, it would be sharp and tangy, with a cheese-like ripeness.
Shop-bought buttermilk varieties are softer tasting — creamier and more mild. They are made by adding a culture to milk, allowing it to ferment in a controlled environment. Though their tastes are very different, the properties of shop-bought and artisanal buttermilks are the same. Both contain enzymes which tenderise meat, making them great for marinades. And both are very acidic, meaning that a reaction takes place when mixed with an alkaline ingredient, creating carbon dioxide bubbles and causing breads and other baked goods to rise.
In the absence of buttermilk, lots of cook books advise adding lemon juice to milk to create a mock-version, which is acidic enough to make baked goods rise. Sales of buttermilk have seen a recent rise though, and the upshot is that it is become more and more widely available. Most decent-sized supermarkets stock a buttermilk in their dairy section — but try a good cheesemonger, or a local dairy like Ivy House Farm for a tangy, artisanal product.