Google Associate

The Google interview process is brutally long (no matter what team you interview for), but highly variable in terms of content and difficulty depending on the team. The advice I was given was that if you want to work at Google, you have to put up with the long process.

Additionally, Google does not hire pools of people in advance -- generally the recruiting happens just a few months prior to graduation when they know exactly what roles they need to fill.

The actual interviews look for:
-- General Cognitive Ability (can you do math, solve problems, etc)
-- Role Related Knowledge (if you're interviewing for finance, can you value a business; for sales, do you have a deep understanding of Google's business model and can you explain our Ads system to your grandmother)
-- Googleyness (do you fit in culturally)

The best way to prep for a Google interview is to get a really good grasp on Google's business model, products, and competitive landscape. Almost all business interviews will involve actual problems faced by the team you're interviewing for, and they know the space intimately. They're looking for someone who is smart and understands what they're getting themselves into.