1) unit of measure, for walkable distances in a city. "How far is that?" - "four corners" (četiri ćoška). Because the idea of blocks applies only to Novi Beograd, where groups of buildings which were built together are officially called blocks. And their sizes and lengths differ, so they are not a unit. Actually, some blocks contain small streets, which are also considered when counting corners.
2) not a curve on a road. When one says "he turned a tight corner there in the switchback in the mountain" everyone tries to imagine the idiot who built a street like that. "Flew into a curve", "cut a curve", that's how it's said here. There's no corner without a building.
3) place for loitering. Most frequently, the kids from the 'hood will gather around any open space that's close to a thoroughfare but not immediately on it. Lacking that, there's always a corner nearby, next to a house with nobody sleeping right behind the nearest window (or else a neighborly dispute may take weeks).