It's a top down space-station simulator shackled with horrible lag and clunky controls. Once you get past that, it's utterly amazing.
There's a space station with all the various departments and jobs needed to keep it running sch as quartermasters laboring in the cargo bay, doctors bringing the sick to medbay, and a security department with a staff of officers. They try to keep the station in working order.
The actual challenge greatly varies: ranging from a blob monster eating the station, to changelings that stealthfully absorb and assume the identity of crewmen, to a revolution trying to overthrow the tyranny of the captain, to a meteor storm, to a direct assault by 6 well armed syndicate commandos.
There are bots in the form of things like monkeys used for experimentation and zombies, but for the most part every person or android you encounter is controlled by a human roleplayer.
Games are played in rounds, generally ranging in time from 30 minutes to an hour. There's a lot of varience in between servers, as they mostly use different code and promote different playstyles.
Because of the crappiness of BYOND there have been many remake atempts, but the only one I'd wager on is located at
http://spacestation13.com/. Keep in midn that there's a big learning curve, and you should read up on everything before you dive in. I hope this post makes sense.