XCOM: Enemy Unknown will seek out the "Enemy Within" when the critically acclaimed reboot receives its first major expansion on November 12. Enemy Within will be available on PC and Mac for $30, while Xbox 360 and PS3 get the $40 "Commander Edition" that includes a copy of Enemy Unknown and the Slingshot and Elite Soldier content packs.
Enemy Within takes place in the confines of Enemy Unknown, so it's the same game with more stuff. That descriptive noun includes new soldier abilities earned through alien research, such as gene mods and MECs, more commonly known as "mechs," even more commonly known as awesome mechanical exoskeletons that include flamethrowers and grenade launchers.
The expansion also adds new weapons and equipment, new enemies – like the "Mechtoid," a machine-encased Sectoid soldier, more resources, additional "tactical challenges" on maps and much more content for multiplayer.