Cross-cutting: An editing technique in which two seemingly unrelated scenes are inter-cut forcing the audience to associate the two
Example: The Godfather
The most famous example of cross-cutting occurs when Michael Corleone stands up at his nephew's baptism. While he is vowing to renounce Satan all the rival mafia bosses are getting offed, we presume, under Michael Corleone's orders. These two scenes are interwoven, contrasting Michael's supposed religious devotion with his ruthless killing of several men (and one mistress).
Cross-cutting can create a sense of tension, can mislead the audience into believing that events that are occurring simultaneously are related when they actually are not, and can relate seemingly disparate events.