The very dry and heavily hopped ale usually served on draft in British pubs is called bitter. Bitter is a pale ale and the more pronounced taste of hops distinguishes it from more mild ales—hence the name. Bitter as a kind of ale is not to be confused with bitters, the aromatic distilled liquor used in mixed drinks.
Old beer is a comparatively unpalatable shadow of its former self—skunky in odor, bitter in aftertaste. But over time some of those bitter compounds degrade into less appealing substances that lend the aged beer a harsh, bitter aftertaste.