If not for the hard work, generosity, and love from talented mappers, this server would definitely not have as many maps as it currently does!
Maps with a icon have full bot support, and the server will exclusively run those maps when there are fewer than four players in-game. A number of bot navigation meshes were sourced from SleepyAl of Facepunch, mdawgmike of the Steam Users' Forums, and many of them (particularly the Workshop ones) are generated and modified myself.
Maps with a blue overlay are automatically synced up from the server's Workshop collection, and will always be up-to-date with the latest version available.
A Steam Workshop link to the map hosted by its official creator(s) is provided in
the map name overlay, so show your support! You can also use the
/workshop command while
in-game to view the current map's Workshop listing (provided it's a Workshop map).
Lots of custom maps have been made over the years; many have not made it to the Workshop officially. For custom maps without a source, your current best bet is currently Google.