Updated: May 1, 2019
If you read any of the NWS discussions (such as the Area Forecast Discussion), there's no doubt that the acronym MCS is tossed around quite a bit during the warm season. MCS stands for mesoscale convective system. These are not just a complex of thunderstorms as they are a "system" of thunderstorms. If you fly during the summer months (most of us do), they will play a major role in your preflight analysis if you fly anywhere east of the Rocky Mountains.
Their typical signature can be found on the color-enhanced IR satellite as a large oval-shaped shield of very cold cloud tops like the one shown below. It's not unusual to see them in pairs. In fact, the clouds at the bottom of this image are convective debris from another MCS that had previously dissipated.
Tucked under these systems is a line of strong to severe thunderstorms that have a bowing structure. As with any bow-shaped line you can expect some strong straight line winds. In most cases either a very strong rear-inflow jet or a strong downdraft deforms the leading active line of convection into a bow-shaped arrangement.