What is the difference between ale and beer?

Both beer and ale are mild alcoholic beverages made by yeast fermentation from barley, but the difference is the yeast used. Ale is brewed by using top yeast (Saccharomyces cerevisiae) while beer is brewed by using bottom yeast (Saccharomyces pastorianus).

Top yeast, as its name implies, floats upon the brewery batch and ferments the batch from top of the vessel, forming an airtight cover, and once all fermentable carbohydrates in the batch have been converted to ethanol, it descents to the bottom of the vessel. Top yeast can also work at any habitable temperature. Bottom yeast ferments the batch from the bottom of the vessel, and the vessel must be closed with a bubble lock to prevent contamination. Bottom yeast is extremely finicky to temperature, and works best between 6 to 16 deg C. Before modern refrigeration technology, beer could be produced only in autumn and spring.

Both types of beverage benefit from storage fermentation. It is called "cask conditioning" on ales and "lagering" on beers. The taste is formed during the storage fermentation.

Ales are usually stronger in alcohol contents than beer and have a wider variety of tastes, but beer is more constant with its taste and usually considered fresher.