Seattleites usually describe Seattle locations in terms of "neighborhoods." This is partly because of a potentially confusing system of street addresses (see Get around). The breakdown into neighborhoods is informal and mutates over time, and while there are often signs on major arterial roads to let you know that you are "entering" a particular neighborhood, the placement of these signs is arbitrary and occasionally controversial.
Still, knowing what neighborhood you're looking for can be a good sanity check when you're looking for an address. A Seattleite would describe 1401 45th Ave SW as being in West Seattle, and 1401 NE 45th St as being in the U-District (University District), which you'll note are diagonally opposite on the map. See Get around for an explanation.
The Seattle City clerk maintains an interactive map  that starts with the high-level districts, but lets you click on those to get the detailed neighborhoods too.