Google is one of those companies that have people lining up to join them. Google treat their employees well and look for the best of the best.
Obviously this means getting a job placement at Google is quite difficult. There are lots of clever people lining up for a job interview and subsequently they have quite a difficult interview process to weed out people they don't need.
Here are some tips by a Google interviewer. These job interview tips could be applied to Google or any other job. Take a look at Get that job at Google.