This article is from the Real Ale FAQ, by with numerous contributions by Brett Laniosh others.
A: Real ale MUST be alive when you drink it. This is the fundamental
definition. The alternative is pasteurisation. (ie killing off the yeast
before the beer leaves the brewery.) Real ale continues to ferment in the
cask or bottle after leaving the brewery. This process is known as
secondary fermentation. As the fermentation proceeds after putting into
casks (cask conditioned) or bottles, (bottle conditioned) the carbon
dioxide produced is dissolved into the liquor and gives the beer a natural
measure of 'Condition' (see section 2.b). If you have killed off the yeast
before casking you have to add CO2 to make the beer fizz. In the majority
of cases real ale will be brewed with traditional (or variations of)
recipes using traditional techniques.