I have an unusual recommendation to make. If you should try out XCOM: Enemy Unknown, try the following settings for your first game: Easy difficulty level, and under Advanced Options select Enable Ironman. If you played a lot of the original XCOM and many tactical games since, you might try Normal and Ironman, but for most people I'd recommend Easy. By enabling Ironman, and thus not being able to constantly save and reload when things go wrong, you will get a real sense of consequences. And using a lower difficulty level instead of the "hidden" way of making the game much easier with saves and reloads will make your first game a lot less frustrating. You can work your way up from that afterwards.

XCOM is a game about consequences. A game about interesting decisions. And the only way to make decisions interesting is to offer the possibility for any decision to turn out to be wrong, and to have negative consequences. XCOM is a great game for that, but for people who weren't around when games like that were made in the early 90's, that can be a bit of a shock. If you are too cocky, especially in the first game where you don't understand all the strategic impacts of your decisions yet, you can easily end up with a "game lost" instead of a "game won" screen. There aren't many games around these days that have an actual loss condition, unless you play against other players. Even more shocking: While some negative consequences are a result of bad decisions, sometimes you lose a soldier just by bad luck. It is interesting to read on the internet how many people have a problem with that concept.

There are two major parts to XCOM: Enemy Unknown: The first is a strategic part, where you plan the defense of Earth against the aliens. Resources are scarce, and you need to make sure that you have both adequate means to detect and shoot down UFOs, and to battle aliens with your soldiers. The latter is the second part, a turn-based tactical game. There are tons of different maps, and different scenarios, from alien abductions, rescue missions, capturing a downed UFO, to invasions of an enemy base. The strategy part looks a lot simpler, but is in fact somewhat more devious: Bad decisions can have strong consequences several hours of gameplay down the road. Thus my recommendation to learn about strategy and consequences on Easy Ironman.

XCOM currently has a Metacritic score of 89, which is unusually high for a turn-based tactical game. Especially since it isn't without flaws: The tutorial sucks with too much hand-holding, which is unfortunate because that is what you get to play if you download the demo. The character animations in the cut-scenes aren't great. And the free move the aliens get when you spot them gives them a rather unfair advantage. But compared to other turn-based tactical games that have come out this decade on the PC, XCOM is probably the best.