So what is changing? In the past, Google Analytics ended a session when one of the following occured:
1. More than 30 minutes elapsed between pageviews for a visitor
2. End of the day
3. Visitor closes their browser
With the update, number three in the above list is modified to end a session when any traffic source changes. For example, if a visitor reaches your website via an AdWords ad and then comes back to your site via a different campaign or traffic source (within 30 minutes), the initial session will be ended. In turn, this will establish a new visitor session.