Applied online at Amazon.com. Got contacted for an interview about a month later. Started with a phone screen with who would be my manager. After about a week I was contacted to do a 2nd phone screen with the next higher-up manager. After some time, I was contacted for an on-site interview loop in downtown Seattle. It was almost an entire full day of interviews (with a lunch break).
The interviews were generally very relaxed. I was asked general questions about my abilities, why I want to work at Amazon and related behavioral questions.
Definitely know the Leadership Principals and tie them in to every answer if possible! Amazon lives and dies by those principals.
Finally I met with HR after about 6 or so interviews, and they explained the benefits and reviewed my salary expectations and explained when I would hear back regarding their decisions.
It appears that Amazon doesn't interview multiple candidates and compares them - they interview a single candidate at at time for a position, and then decide yes or no - then move on to the next one.