Lengthy and lots of interviews - I had about 7 or 8 total including calls, and face to face interviews.
They ask many questions about your experience and situational expertise and reactions.
One of the things they try to do with such a lengthy process is get a good feel for you as a candidate overall. IT also allows the candidate to get a feel for the company and the culture to see if it is a good fit as well.
You will interview with HR, your line manager, your colleagues and peers sometimes, and other managers you'll be working with as part of your job.