My cat walked across my computer and somehow made all of the e-mail in my main account become grouped by sender such that the expand button must be selected to see all of the e-mail by a specific sender.  Only the most recent e-mail by that sender is show while the others are either hidden or when expanded, shown under the most recent e-mail from that specific sender.

While it is a nice feature, I would like to know how to turn it off and back on later.