One of the best games ever made
I don't toss that around lightly. XCOM UFO Defense was, and still is in my eyes, one of the most engrossing real time simulation/tactical turn based strategy games ever made. It has yet to find an equal, which means going back and playing the original game, unmodified, is actually not that painful an experience. Going to anything beyond the original, well, your mileage may vary, but I'm not here to talk about the many terrible games that XCOM spawned, but instead the beautiful synergy of the original release.
Starting the game up, you select your difficulty (Beginner, unless you are a madman), and you select your starting base position. You could put your first base anywhere in the world, with varying degrees of landmass covered. You can have multiple bases, but the ones beyond the first cost money. Money is your basic currency, and is used to pay for equipment, buildings, troops, scientists, manufacturing, ships, etc. etc. Makes it easy to keep track of when you just want to have a bunch of money. Your base serves a vital function in the defense of UFOs, housing all your troops, equipment, facilities, strike craft, and so on. There's a lot under the hood of XCOM, and it's surprisingly well designed for a game it's age.
The two different screen modes are basically two seperate games, so far we've been talking about the Geoscape, but eventually you are gonna detect some UFOs buzzing around and it's your job to shoot em down. Once a UFO is downed or landed, you can send in a troop transport and switch to the other gameplay mode, which is a tactical turn based strategy experience. It's very tense and requires strict monitoring of Time Unit consumption and cover, doors, flanking and supporting positions. The early small UFOs aren't bad, but eventually you will get called upon to defend civilian towns being terrorized, and eventually even alien bases established to counter your efforts. Failure to respond to these threats means the country with the alien activity will send less money, hurting your overall efforts. They can even be infiltrated by the aliens and pull out of the pact entirely, letting the aliens establish bases freely in their territory.
XCOM is a game that's hard to describe, and extremely difficult to beat. I've gotten close in recent years, but as a child I never even scratched the surface. The whole series is available on Steam now, and the first game alone is worth it at twice the price. The rest can serve as an interesting side step into the decline of a brilliant game series.