It is an open source MMO RTS sandbox game for programmers, wherein the core mechanic is programming your units AI. With all the attributes of a full-fledged strategy game, you control your colony by writing real JavaScript which operate 24/7 in the single persistent world filled by other players on par with you.
Your colony can harvest resources, build units, conquer territory, trade with another colonies. As you conquer more territory, your influence in the game world grows, as well as your abilities to expand your footprint. However, it requires a lot of effort on your part, since multiple players may aim at the same territory.
Screeps is developed for people with programming skills. Unlike some other RTS games, your units in Screeps can react to events without your participation – provided that you have programmed them properly. And, unlike other MMO, you do not have to play Screeps constantly to play well. It is quite enough just to check once in a while to see if everything goes well.
Features:
You play by writing JavaScript AI which controls your units 24/7 even while you're offline.
Units of all players coexist in the same real-time huge persistent world and obey the same rules.
Many kinds of units, base building, mining resources, territory control, economy, manufacturing, transporting, logistics, trading – all the attributes of a real strategy game which you need to program!
The better your scripts, the better your game — irrespective of the time played. Your creeps will mine, build, defend, and conquer as you just work, sleep, or walk your dog.
Only basic programming skills are required. However, if you are a pro developer, now is the chance to put your skills to the limit!
Edit your scripts from the in-game editor, or using your favorite external IDE.
Lifetime MMO access with 10 CPU limit included in the starter package. Raise your CPU limit to the point depending on your game level by purchasing a CPU subscription plan or via in-game items called Subscription Tokens. The subscription cost is the same for all players, there is no way to pay more to gain more advantages.
Host your own server, modify games rules and play with your friends via LAN or on the Internet. No subscription required. The server is an open source project.
And many more features to come! (see Roadmap)

Screeps videos