As far as a lot of casual beer drinkers are concerned, pilsner and lager are pretty much the same thing. This is likely because the only lagers they have ever had were pilsners or a style they thought were pilsners (mor on that later). Either way, it’s not true. A pilsner is one of many styles of lager [link to Ales v. Lager article]—that is, bottom fermented and conditioned at cold temperatures—but the terms are not interchangeable. All pilsners are lagers, but not all lagers are pilsners. But for those who prefer not to split too many hairs, pilsner and pils are interchangeable. But let’s start from the beginning.
The History
Pilsner (pilsener is also correct, by the way) is named after Pilsen, a city in Bohemia, in the Czech Republic. In 1838, the story goes, the citizens of Pilsen dumped a whole season’s worth of beer in front of Town Hall, and not just for their fallen homies. Rather, they were dissatisfied enough with the quality of their beer that they did it as a way of demanding better quality standards. There aren’t too many non-violent acts that demonstrate one’s sincere displeasure quite as powerfully as dumping the season’s beer supply.
As a solution to bad beer, those citizens (it is tempting to call them Pilseners, but that would muddy the waters a bit) built the Bürger Brauerei (Citizens’ Brewery). Bavarian lager was all the rage at the time, so Martin Steltzer—who spearheaded the Bürger Brauerei effort Josef Groll—a Bavarian brewer—to be the brewmaster. Groll employed Bavarian assistants and yeast to make a lager in the Bavarian style.
And that he did, but it was a markedly different lager that resulted. The yeast, barrels, and know-how were Bavarian, but the ingredients were Czech, and that made all the difference. The soft water, as well as the local barley and hops, gave the Czech pilsner a character all its own. Not surprisingly, it very quickly became a sensation.
Oddly enough, though, the citizens of Pilsen neglected to trademark the pilsner name until almost twenty years later, by which time the name was used by many other brewers. In 1898, the Bürger Brauerei trademarked Pilsner Urquell (“original pilsner”), which it brews to this day.
Styles of Pilsner
Depending on whom you ask, there are two classic styles of pilsner—plus everything else. They are as follows, along with a few examples that you have probably heard of:
Czech (aka Bohemian) – Pilsner Urquell, Sam Adams Noble Pils, Sierra Nevada Summerfest Lager, Lagunitas Pils
German – Beck’s, Warsteiner, St. Pauli Girl, Brooklyn Brewery Brooklyn Pilsner
Everything else (which some call European Style) – Stella Artois, Heineken, Amstel

A word of warning: although they are often confused by the casual drinker, the difference between the pilsners in the aforementioned classic styles and American pale/light lagers (that is, lagers that people think are pilsners, but are not) cannot be overstated. These include the big names: Budweiser, PBR, Miller, Coors, Labatt, Molson, etc. These are also known as adjunct lagers because of their use of corn and rice adjuncts in the brewing process. They all have their place, but they are not big on flavor. Pilsner does have plenty of flavor, so the point here is to be careful not to confuse the two. Or at the very least, don’t mistakenly judge pilsners based on the adjunct lagers.
From here, the best thing to do is to grab a handful of each style and taste/see/smell/feel for yourself what the difference is.