How do City Council and Mayoral Elections work in Portland?

I was having a conversation with a friend yesterday and the topic of the 2012 Portland Mayor’s Race came up.  As we were both non-native Portlanders, we tried to piece  together an understanding of how local elections worked, as if the answer lay in a collage of how it works in other cities.

City Council seats, as well as the City Auditor, are non-partisan, elected positions; each carries a four-year term. As with all non-partisan positions in Oregon, candidates face off in a primary election (typically in May of even-numbered years); if no candidate wins more than 50% of the vote, the top two finishers face off in a runoff election (typically the following November.)
Source: Wikipedia

Turns out the Mayor’s seat, along with the rest of City Council and the City Auditor are “non-partisan” positions.

How do they do that?  My best guess is through the city’s charter, but I haven’t found the answer yet.