XCOM: Enemy Within is an expansion for XCOM: Enemy Unknown, a turn-based strategy game where players defend Earth from an alien invasion. The expansion introduces new content, including additional missions, maps, enemies, and the “Meld” resource, which allows for new technologies and soldier enhancements like genetic modifications and mech suits. It also adds a new faction, EXALT, which brings additional challenges. The game focuses on tactical combat, base management, and strategic decision-making.