A CUSUM (CUmulative SUM) control chart is a specialized control chart, used to detect small and systematic changes in the process mean. It differs from a conventional Shewhart control chart in that each new point plotted on the chart is a cumulative sum of the deviations of the previous Samples from the mean or target value.
The underlying principle of CUSUM charts is that small shifts can be detected more easily when information from past data is consolidated.
The advantage of this chart is that it is more sensitive than Shewhart charts in detecting small, sustained shifts (between 0.5 and 2 sigma). The disadvantage is that it is less sensitive to large, abrupt changes in the mean and also slower to detect special patterns in the data. For this reason, it is often recommended that CUSUM charts be used in conjunction with Shewhart charts, to maximize the benefits of control charting.
From the NIST webpage: - http://www.itl.nist.gov/div898/handbook/pmc/section3/pmc323.htm