So you build stuff and then what?
Then whatever you want...
There are different sorts of servers and maps. Some people start a randomly generated world and try to survive in it (you can set it up with or without roaming monsters that attack you), some people just make artworks and giant sculptures, there are "challenge maps" that are hand created to have some sort of goal to accomplish such as collecting a specific resource from a hard to reach location.
Depends what kind of experience youre after whether its suited to you or not. Do you want to just hang out and be friendly with people, form friendly rivalries, or have serious head to head competition?
Its a bit of a tough question, kind of like just saying "Id like to read a book". Im having trouble thinking of games that are on Steam, have a low-ish barrier to entry (ie, suited to someone who isnt a hardcore gamer), and have a well regarded multiplayer.