Delta and Alaska Airlines have been partners for years. Alaska's huge west coast route network fed into Delta's huge international and transcontinental network out of Seattle. They each honor the other's elite status and you could earn miles on one for flying on the other.
Earlier this year, though, things started to go sour, and it's pretty clear Delta is bullying Alaska into allowing themselves to be bought up by their former partner. While this drama unfolds, it's probably a great time to find cheap fares and lots of bonus point opportunities on routes where they're competing (pretty much Seattle to anywhere in the western US and DC/NYC).