The word ale has followed a movable pattern over the centuries, as the following extract from the OED will illustrate.
The enduring characteristic of the word is that it has been employed in opposition to beer to describe a more traditional variety. From the 15th century ale was beer that had not been brewed from hops, but from conventional malt.
But has it now reached the point where we have a settled definition? In other words, are there any claims on the word ale other than the cask-brewed real ale, sold on tap, with no added pressure? This is as opposed to mass-produced nitrogen or carbon-dioxide-added beer that is sold in cans and some bottles. http://en.wikipedia.org/wiki/Cask_ale
With the introduction of regular use of hops into England from at least the 15th cent., ale came to designate beer brewed without hops, a distinction which lasted until the 18th cent., when it came to refer to a strong beer. In the 19th cent. ale was sometimes distinguished by its being brewed with malt that has not been roasted or burnt, and hence being paler in colour. The word is now most common (outside historical contexts) as a trade name, or (esp. in real ale, cask ale, etc.) referring to traditionally brewed beer (of any colour).