UFO - Enemy Unknown (or _X-Com: UFO Defense_, as it is called here in the U.S.) ... really is hard to describe accurately. I can and can't recommend this game. It takes hours of dedication, but once you get into it you will surely dedicate thousands of hours of your life to it.
The music is extremely beautiful; a combination of military beats with "techno" melodies to merge Earth-based music with "futuristic" music that might appeal to a Martian. You fight about 6 different hostile alien races: the famous Zeta Reticuli race is here (called "Sectoids" in the game); there are reptilian aliens ("Snakemen"); creepy death look-alikes with awesome mental powers ("Ethereals"); and tough jock-types ("Mutons").
You have armored vehicles at your disposal (but so do the aliens), and even though you start with puny Earth-weapons; as you shoot down, recover, and research alien technology you can build base facilities, aircraft, tanks, weapons, body armor, grenades, EXTREMELY high-explosive missile launchers--and even equipment for MIND CONTROL--based on alien technology.
At the end of each month, you're given a progress report. Nations decide if they should increase your funding, decrease it, leave it the same, or pull out of the alliance all together and try to strike a deal with the alien races.
The ground battles themselves are played in a chess-like state. The "board" is divided into (I think) 124x124 squares; each one square taking 4 "Time Units" to cross. The average soldier starts with 50 TU's. However, shooting takes between 15 & 45 (depending on weapons used & type of shot--aimed, auto, snap etc.). Basically, everything--reloading, throwing stuff, turning your soldier's head--takes TU's. But, as your soldiers see more combat, they can get as much as 81 TU's, which is quite helpful. They also gain health units, stamina, strength, bravery, etc--but keeping a soldier from the first mission to the final assault on the main ET base (in the Cydonia region of Mars) is pretty much impossible. You're fighting a well-trained enemy!
Each group of 25(?) squares is given it's own unique attribute, so you'll NEVER (I, in 6 years of playing had not) come across the exact same map twice. The aliens are positioned randomly through the map as well.
There are a few videos in this game, the two outstanding ones are when you win the game. The other being when you lose. When you win, no indication is given of Earth's future state. Do we achieve peace on Earth? Do we forget all that we've been through & return to our old ways? Unsure. You just see the Mars base exploding, and your brave soldiers flying back to Earth cheering. Then you are returned to the main menu. Did the designers intend to create such a profound ending?
The video when you lose is even more mind-boggling. Three alien battle-ships hover over the UN building. Inside, the floor is bare of representatives except for a few humans sitting at a table across from two "Ethereals" and a "Sectoid" commander. There is no talking, but the message is chillingly beautiful in it's clarity: most of the world has given in, and it looks like the last fighting nation is following. As the humans & aliens communicate telepathically, several aliens burst through the door, place a plasma rifle to the head of the human leader, and pull the trigger. The "sectoid" across from him is seen splattered in the man's blood. Proof that the aliens do not make deals--deals, in the sense that we know them--and a taste of what the aliens have always had in store for us: extermination.
This game is no longer in print, apparently because people saw it as "too intelligent." If you happen to find a copy on ebay...consider buying it. I make no promises that you'll like it. The "others" tell me it's a boring & tedious game. I consider it an highly intelligent work of art; one that shaped my teen years. I will always cherish X-COM.