Cities: Skylines is a game with a modern take on the traditional city management genre first introduced by SimCity. The game is a newcomer and has a significantly smaller scale than its predecessors, yet it manages to stand on its own.
The core gameplay is familiar to any veteran of the genre. The player takes control of an in-game city and develops it maintaining the elements of infrastructure: building roads, raising buildings, opening and maintaining metropolitan facilities, etc. You can't go as big as some of the other games in the genre, but the level of detail is impressive. The city is divided into areas with their own specializations, the state and characteristics of those areas can change depending on your management. Just like a real town, your town can go into decline or prosper, be an industrial heavy-loader of a tourist attraction, and many other things.
One advantage I really like is that Cities: Skylines changed the interactive element of building mechanics. Now it's not big square sectors, but more realistic construction forms allowing for building something in the triangle or a smaller square area, etc. It really adds to the comfort and atmosphere.
Another entertaining thing is the simulated population reacting to your management on funny in-game social media, sometimes providing useful tips and advice on how to manage the city better. The level of detail is, again, compensating for a smaller scale of the gameplay. Your people have names, jobs and attitudes.
All in all, Cities: Skylines is a neat entertaining game, building up on the legacy of bigger, but sadly forgotten or obsolete titans of the city tycoon genre.