An Open Tibia server — an OT server, OTS or otserv — is a Tibia world run by its own community rather than by CipSoft. It runs open-source server software that speaks the same protocol as the official game, so an ordinary Tibia client can connect to it once it is pointed at the server's address.
Everything the official game fixes is a decision on an OT server: experience rates, PvP rules, which client version is supported, what the map looks like, which monsters and items exist. 110 servers are listed here across 11 countries, and 76 answered the most recent status check.
That freedom is the appeal, and it is also why a list like this has to exist. Servers open and close constantly, so the useful question is never which one existed last year but which one has players on it tonight — which is what OTServers.org measures rather than asks.