If you use Google Analytics, this post (and accompanying video) is a must-read. Avinash, an expert on web analytics, especially Google Analytics, explains how the session algorithm changed and the resulting impact this has on various reports and metrics in GA.

Of particular importance, this changes the number of visits (not unique visits, just visits), as well as that metric’s cascading change on avg pageviews per visit, time on site, bounce rates, etc.