Video games that fall into the 4X category follow a very specific pattern; they are based around the following four concepts: Explore, Expand, Exploit, Exterminate. This is typically used to refer to empire-building games, such as Age of Empires, Rise of Nations, or Sid Meier's Civilization.
This is where you explore the territory around you, searching for natural treasures and possible allies.
This is where you expand your influence to gain access to new resources.
This is where you set up the infrastructure to take advantage of strategic resources.
Using your resources, you wipe out your opponent(s).