Sums widgets display the calculated sum of the values of a certaing metric for a given period. Those widgets include a trend as well.
Sums widgets are only available for the following metrics: Users, Sessions, New users and Exception occurences.
We will use the Users metric for this example. If a date period of [May 16, 2014 - May 22, 2014] is selected then the widget will show the total number of users for this period by basically calculating the sum of the users for all days in that period.
Sums widgets display a percent formatted number called trend. It shows the difference between the value calculated for the currently selected period and the previous period with the same length. So if we select the same period as the example above then the previous period with the same length would be [May 09, 2014 - May 15, 2014]. The trend shows how many percent the total number of users has increased or decreased between the two periods.