Player controls XCOM (eXtraterrestrial COMbat unit)- an international task force created to fight alien invaders. In order to resist the invasion, player has to build and manage bases all around the world, research new technologies, intercept UFOs and fight the greenskins on ground in turn-based combat.
Every structure in ground combat can be destroyed. If you are allergic to doorknobs, feel free to blow away the whole wall to enter the building and take the alien tourists by surprise.
Take the aliens` equipment and research it in order to discover new and exciting ways to blow them to bits.
Your funding depends on your effectiveness in the region. If you neglect China and let the saucers buzz in its skies freely, they`ll eventually cut the funds and might even become allies to alien overlords.
Game is largely free form, meaning, there is no set mission structure - maps are created randomly and each game is different.

• IGN review of X-com: UFO Defense
• Xcomufo-for all your Xcom needs
• Xcom: UFO Defense Wikipedia entry
• Review at Twenty Sided