Only One Pilsner
You do not order a "pilsner" in Prague (or anywhere else in Czech). You could order a Pilsner, though. In the Czech Republic, the word pilsner is a proper name reserved for Pilsner Urquell. All other pale lagers are referred to by either their proper name or by category (see below). I have gotten several different answers for why this is the case, but my sense is that it has mainly to do with tradition. Josef Groll invented pilsner at the old burghers' brewery in 1842, and other breweries show great deference to this brewery (now called Plzensky Prazdroj, or Pilsner Urquell). That beer is the ur-Pilsner, the one that begat the rest. It is also the beer from Pilsen--not the only one, but obviously the big one--and so for these reasons it is the only one people call pilsner.
The Categories of Beer
° pale beer or a 12° amber or a 14° dark. But you might also have a 12° dark. (On our tour, Evan Rail mentioned that while there are no hard and fast rules, if you see a brewery list that includes a 10, 12, 14, and 18, the average Czech would assume the two smaller beers are light, the two bigger ones dark.)
Let's start with the legal designations, which refer to Plato categories. These changed a bit in 2011, so if you find lehké on an old list, note the change. Also, those are my best-guess pronunciations you find. Fluent Czech speakers may offer corrections or denunciations in comments.
Update: Indeed, the wisdom of hive mind is speaking loudly in comments, with corrections, questions, and clarifications. Definitely have a look.
- Stolní pivo, table beer up to 6° P. (I've never seen one of these in the wild.) The pronunciation is roughly stole nyee Pee voh.
- Výčepní pivo, from 7° to 10°. Strangely, výčepní comes from the word for taproom and the term literally means “draft beer.” It is applied to all beer in this range, irrespective of package. Pronounced vee chep nyee Pee voh.
- Ležák, from 11° to 12°. Again, to add to the confusion, ležák literally means lager—and again, it applies to all beer in this range whether lager or ale. Pronounced leh zhak.
- Speciál, strong beers above 13°. Pronounced spet zee-al.
- Světlé, or pale-colored. Pronounced svet lee.
- Tmavé, or dark. Pronounced t’ma veh.
- Polotmavé, which literally means semi-dark or half-dark, referring to a color in the amber band. Pronounced polo t’ma veh.
- Černé, or black. Pronounced cher neh.
Bright, Unfiltered, or Yeasted?
So far, so good, yes? Now comes the more tricky part of the whole thing. Not only do you have this taxonomical tangle, but you have an additional stratum of information regarding how the beer was prepared. In addition to just regular old beer like you might find in a bottle, the beer might be unfiltered or served kräusened.
- Kvasnicové, literally yeast beer. It is a specific preparation that involves adding yeast or fermenting wort to fully-lagered beer right before kegging. It brings a liveliness to the beer that has Czech beer geeks in a swoon. Pronounced kvass nitso veh Pee voh.
- Nefiltrované or unfiltered beer. Slightly confusing because both kvasnicové and nefiltrované will appear less than perfectly clear in the glass, and both may enjoy the benefits of richer, brighter flavors. Unfiltered beer is not kräusened. Pronounced ne filtro vanay Pee vo.
- Tanková, or tank beer. Just means it's served from a large, 5- or 10-hectoliter tank underneath the bar. What's significant is that this beer is unpasteurized, which means the flavors are sharper and more vivid. Pronounced tank o va.