NYC has five boroughs. These are also counties. the borough of Queens is Queens County, Brooklyn is Kings County, Staten Island is Richmond County, etc. Within these five counties are numerous neighborhoods. They correlate to certain zip codes, for the most part.
Do this: Go to maps.google, and type in "Seneca Ave, NY." That puts you right in the middle of Ridgewood. Zoom out times one, and you'll see all the neighborhoods named on the map. You can easily see Ridgewood, Glendale, Woodhaven, and Bushwick. Many neighborhoods are like a melting pot, but others are clearly segregated.
Just walk through Brooklyn. You'll find mostly Polish in Greenpoint, Hasidic Jews in parts of Williamsburg, and Puerto Ricans on the other side. You've got Italians and Irish down in Bay Ridge, Russians in Sheepshead Bay, Blacks in Bed Stuy, etc. That's not to say that any one area is better or worse then the next, but that's predominantely what you'll find in those areas.
When I worked in Jackson Hts/Corona, Roosevelt Ave was mostly Mexican, 37 ave from 76th st to Junction Blve was mostly Colombian, Ecuadorian, and Peruvian. Past Junction was Dominican. East Elmhurst was black. 73-76 st was mostly Indian. Many neighborhoods are quirky like that. Astoria can have Greek areas, Brazillian pockets, South American, Paki, Egyptian, Italian, Black, etc. You walk 5-6 blocks in any direction and the scenery changes.