When a Google Nexus phone comes out, it's usually the best Android phone available at that time. The reason for that is simple: Google gives whoever makes the Nexus phone access to the latest version of Android, meaning other manufacturers are left scrambling to catch up. According to the Wall Street Journal, though, those days may be over.
Now, Google will let multiple manufacturers make Nexus phones at the same time. So instead of only one Nexus phone being availble every year or so, multiple Nexuses should hit multiple carriers over a shorter time frame.
By expanding the Nexus line, Google will be able to propogate new versions of Android more widely. Customers will also be able to choose from a variety of top-of-the-line Android phones made the manufacturer of their choice.
As with previous Nexus phones, Google will sell the new models directly to customers online (most likely via Google Play's new "devices" section) and with option of purchasing them unlocked.