Ale is fermented using Saccharomyces cerevisia, a very common yeast type used for a number of different applications. Though there are many subtypes within the ale category, broadly speaking, ale is fermented at higher temperatures and results in fuller flavours and higher levels of alcohol than you would find in a lager.
Ale is referred to as top-fermenting because the yeast will first rise to the top and then sink to the bottom of the brewing vessel as the fermentation is finishing.