A strategy that can be used for more easily defeating a large number of opponents. The idea behind this is that defeating a large number of enemy forces at once is hard, possibly impossible depending on the circumstances, but if the large force was to be divided into smaller forces, each one may be taken on individually and thus more easily. A couple famous conquerors who have used this strategy are Julius Caesar and Sun Tzu. A simple example:
Bob and John want to defeat Killer Robots A, B, C, and D. The two of them are no match for the four robots working together so, Bob creates a disturbance to lure some of the robots away. Since the robots only see Bob, they don't all go after him. Instead, Killer Robots A and B go. Bob leads them to John and together the humans are evenly matched against the Killer Robots. Now they have a better chance of victory. They could have an even greater chance if they were to further split up the robots somehow so they could attack each one individually (i.e., Bob and John versus only Killer Robot A, then only Killer Robot B, etc.).
This strategy can backfire, however, if two separated forces happen to be on either flank of an enemy. The two forces can come together, trapping their enemy in the centre, giving them even more of an advantage.
Another method of dividing and conquering would be to set two or more opponents against one another, thus not having to deal with them as a whole. Dividing their unity and then conquering them in their weaker state. The divide and conquer strategy isn't exclusively used in warfare. Dividing up a large task into several smaller tasks can more easily allow one to "conquer" or accomplish the goals.
In Latin, divide and conquer is divide et impera (this is closer to divide and rule but the same meaning is there).
Thanks to sauron for letting me know about the Latin and Gorgonzola for mentioning examples of leaders that have used the strategy. | 
