Seattle's road designations make sense once you understand them but, if you don't understand them, you can end up many miles away from your destination.
North-South roads are labeled "Avenues" while East-West roads are labeled "Streets". The city is roughly divided into a semi 3 by 3 grid with 7 directional sectors (NW, N, NE, E, W, SW, and S). Street addresses are written with the sector before the name, e.g. NE 45th Street or just NE 45th. Avenue addresses are written with the sector after the name, e.g. 45th Avenue NE or 45th NE. "Avenue" and "after" both start with "a".
Another way to remember avenues: University Way NE, the main street through the city's University District (neighborhood) is called "The Ave" by the locals, and all avenues run north-south. But, don't confuse University St with University Ave; they're two completely different streets!
"Ways" are long diagonals, "Drives" are long, circuitous routes, "Courts" are one block long.