Space Station 13 is a Multiplayer Online Sandbox RPG (Note that it is not an MMO) that is based around the running, and eventual ruination, of the eponymous Space Station, with rounds lasting anywhere from 30 minutes to 3 or more hours (This largely depends on how RP-focused the server in question is and what traitor is active, with malfunctioning AI-rounds generally taking much longer than Revolutionary uprising rounds, for example).
Since many of the bigger servers develop their codebase, ruleset and map independently of another, some features, events and playstyles are unique to specific servers.
In virtually all cases however, certain things stay the same:
Every player starts out with a specific job From a selection of around 30, different jobs have varied levels of responsibility, perks and permissions, starting with the lowly Assistant (Who has extremely low access to the Station, but is also not expected to do any vitally important jobs, and can often get away with hacking into unsupervised rooms to steal anything not nailed down), going to things like Station Engineers and Atmospherics Technicians (required to set up power systems and repair station damage, and handle life support as well as fight fires), Security Officers, Medical Staff, Researchers of various kinds, on to the Heads of Staff and finally the captain and station AI.
Furthermore, every round will have one or more traitors, different kinds of traitors have different abilities (A syndicate agent will have access to a few highly illegal items like powerful weapons and special hacking equipment, while a Changeling can transform into the forms of people they have absorbed and use special attacks to paralyze or otherwise incapacitate their victims, for example) and every traitor has his or her own mission, such as assassinating a specific target, or stealing a particularly rare and valuable item).
Due to the very cumbersome UI, and the large variety of tasks every job, and in particularly the traitors, may have to perform to succeed (And the consequences if they don't understand their jobs, such as Station Technicians causing a complete power failure, or accidentally -or deliberately- unleashing a singularity on the station), this game has a rather steep learning curve, and it is strongly recommended to visit the Wiki first (For example, this is the guide to one of the responsibilities of one of the medium-level jobs).
However, it is a very unique experience that can, occassionally, create amazing stories of unbelievable victories against the odds, or (more likely) hilarious failures on all fronts.