A sirok is a Slavic treat not unlike a small bar of cheesecake, except it is usually glazed in a chocolatey layer akin to those on ice cream bars, and often has some sort of fruity jam or other pastry innards as a filling. They're absolutely delectable. They're best kept refrigerated until eaten, as they are at their most tasty when served cold—but not frozen!
The filling flavours range from plain, to strawberry, blueberry, lemon cream, caramel, condensed milk, and just about anything else delicious and freezer-treat-friendly. Some have wafers or large rectangles of jelly as bases, as well, which adds some more texture to the treat. The cheescakey inside itself is probably closest to vanilla cheesecake, though with a little less obvious flavour, so that it is left to the filling to provide the sweet taste.
My absolute favourite kind of sirok would have to be the Kartoshka variety. See, there's this one Russian confection that at first may appear to look almost like a potato, called a kartoshka. Of course, when you bite into it, you find that it had actually was a core of some sort of chocolate-and-pine-nut filling surrounded by a marzipan "potato skin". This was adapted into a sirok by taking that filling, and modifying it a little bit so that it would be better suited to sirok-handling conditions. The resulting taste is like an odd, pasty sort of creamy, but in a good way. It feels like it should, by the power of your saliva, dissolve in your mouth like a bit of ice cream might, but it doesn't, and you are left with this bloblet of chocolate-but-different to suckle on and bite and coax chocolatey goodness out of. But of course, what substitute are words for taste?
The most common exporter of these amazing treats is Rostagroexport, though there could be other brands as well, depending on region and store chain. Being a Slavic treat, they can very likely be found at Eastern European specialty stores near you, such as Starsky. Try asking your neighbourhood quirky Polish kid. Swing by often, though, and learn which ones you like quickly: they run out fast, because this deliciousness can't be kept a secret. And see if you don't end up liking any of the other ethnic items at the store—I can personally vouch for the various brands of cookies that line the shelves, and my hardcore Russian parents really praise the delicatessen.
Sirok is, as you may have guessed, a romanization—in Russian, it is сырок. By Russian pluralization, сырок becomes сырки, which in Latin letters would be written as 'sirki'; 'siroks' would be incorrect. But English is weird with its word assimilation habits, so it doesn't matter too much.