Ask a brewer what separates an ale from a lager and the answer is short: the yeast, and how warm you keep it. Colour, strength and bitterness have nothing to do with it. A black stout is an ale, a black schwarzbier is a lager, and a pale golden beer can be either.
Yeast is the living thing that turns sugary malt water into beer. It eats sugar and produces alcohol, carbon dioxide and a long list of flavour compounds. Which yeast you use, and how warm the tank is, decides how many of those compounds end up in your glass.
Ale yeast is Saccharomyces cerevisiae, the same species that makes bread rise. It likes it mild: most ales ferment at roughly 15 to 24 °C (60 to 75 °F), though sources quote slightly different edges. Lager yeast, Saccharomyces pastorianus, is a hybrid of ale yeast and a cold-tolerant wild relative, S. eubayanus. It works best at about 7 to 13 °C (45 to 55 °F).
Ale yeast is often called top-fermenting because the foamy activity shows at the surface, and lager yeast bottom-fermenting. That is a handy habit, but modern strains blur it. Temperature is the cleaner rule. Drag the slider to see it.
When yeast works hard in warm wort (unfermented beer), it releases extra by-products. The best known are esters, fruity aromas like banana, pear or apple, and phenols, which smell of clove or pepper. A Belgian ale or a German wheat beer is meant to show them. A cleaner pale ale or stout is usually kept toward the cool end of the ale range so they stay quiet.
Lager yeast fermenting cold makes far fewer of these compounds, so the beer tastes of malt, hops and not much else. That is why a good lager is described as clean or crisp. There is nowhere to hide: small flaws show, which is partly why lagers are harder to brew well.
Too hot is a problem for any yeast. Past the top of its range it can make fusel alcohols, harsh solvent-like flavours that taste hot and headache-ish. A few styles, such as saison, use special strains that thrive far warmer than ordinary ale yeast, so the ranges are a rule of thumb, not a law.
Lager is not a colour or a strength. It is a yeast and a long, cold wait.
The word comes from the German lagern, "to store". After the main fermentation, a lager is moved to near-freezing temperatures, around 0 °C, for at least a few weeks. Yeast, proteins and rough flavours settle out, and the beer turns smoother and brighter. This is why a lager usually takes a month or more from start to glass, while many ales can be ready in two to three weeks.
Ales can be conditioned cold too, and some strong abbey ales are matured for weeks. Cold storage is a tool, not the property of one family.
Lager yeast is a historical accident. Brewers in Bavaria stored beer in cold cellars and, under a 16th-century ordinance often dated to 1553, were barred from brewing in summer. Over generations the yeasts that survived cold conditions were the ones kept, and today's lager yeast is the result. Scientists only identified its wild cold-loving parent in Patagonia in 2011.
Several famous styles refuse to pick a side. Kölsch and altbier from Germany are fermented with ale yeast at the cool end, then cold-conditioned like a lager. California common ("steam beer") uses a lager yeast at warmer ale temperatures. So the honest rule is: the yeast sets the family, and the temperature sets the character.
You do not need any kit to use this, only a better question when you read a menu or a label.