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.
[Trailer] Screeps - MMO sandbox strategy game for programmers
Screeps is a fun game and has a unique angle to it. While it still has some room to grow, it's moving in the right direction. Using actual programming instead of a pseudo-language actually allows the players to focus on better coding habits and makes practice fun. There is a very active community of people on the game's Slack channel so it also has a touch of the MMO chat to it for the social programmers out there.