Customizable card games are generally complicated beasts. There's the 500-lb gorilla, Magic:The Gathering, which has dozens of sets and thousands of cards. Theme games like those based on Tolkien's Middle Earth go through contortions to match the theme, adding all kinds of crazy rules and card text that you need a magnifying glass to read. When Blizzard created Hearthstone, they went in the opposite direction.
Hex resources). Decks are only 30 cards, down from the 60 used by many TCGs. Everything you play is on your turn, so no waiting on your opponent to respond to actions. There's a 90 second turn timer, ensuring that games move along quickly. Cards are all either specific to one class or generic to all classes, limiting the available card pool once you've chosen which class to play.
There is still plenty of complexity in Hearthstone, building on that simple foundation. Each of the nine classes has its own unique feel, implemented through class-specific cards and a hero power, which is available once each turn. Hunters do direct damage, priests heal, etc. There are subclasses of minions (Mechs, Beasts, Murlocs, etc) with various abilities that affect only that subclass, so you can build themed decks around them.
The look and feel of Hearthstone is one of my favorite things about the game. The cartoon-ish style will be familiar to anyone with World of Warcraft experience. You'll hear comments and the occasional hint from the innkeeper and various heroes as you move around the game's interface. Many of the minions have their own voices and will speak up when you put them into play or as they attack. The four corners of the game board are filled with decorative items of various kinds, some of which will react when clicked...a fun distraction while waiting for the opponent to take their turn.
There's a lot to explore in Hearthstone beyond simply playing games against random online opponents. There are solo adventures where you play a series of games against special boss opponents, often with strange and powerful abilities that you wouldn't face against another player. There's the Arena, a limited format where each player chooses cards from a random selection, and plays the resulting deck until accumulating three losses. And the Tavern Brawl, a format that changes weekly and can be anything from almost completely random decks to playing as one of the adventure bosses.
Hearthstone is a free-to-play online game, and in my opinion, has an excellent business model. Every day you get a quest, usually something like "Win 3 games as a Hunter" or "Play 20 minions that cost 2 or less". From those quests you get gold rewards, which can be spent on cards or entry into the limited-format Arena mode. You can add some extra gold and/or cards from winning games or completing one-time tasks. If you play Hearthstone for an hour or so every other day, you'll usually be able to complete 4-5 games and complete 1-2 quests, keeping the gold coming in without spending any money. If you'd rather gather gold or cards more quickly, that option is available via the real-money store. And if you get extra cards, they can be turned in for "dust" that is used to craft specific cards to fill holes in your collection.
I've been playing Hearthstone since it was released in 2014, and never felt that I needed to spend money...although I did put in a few bucks because I enjoy the game so much. Of course, that's playing just for the fun of it, and not trying to be a competitive constructed-deck player. It's a rare talent that can climb the ranked-play constructed ladder without the good epic and legendary cards, and getting those requires spending cash (or inordinate amounts of time, I suppose). If you're just in it for the occasional fun game, though, Hearthstone is easily playable for free.
Hearthstone has become one of those rare games that I've played every couple of days for months, with only a few minor breaks. Usually I'll get tired of a game after a few weeks of regular play, but Hearthstone keeps me coming back with regular updates and interesting formats. Highly recommended for any gamer, even if customizable card games aren't your thing. Hearthstone is different enough that it may surprise you.