So... Well, to make a quick explanation, Tibia is a game, an MMORPG with 2D art, that's been online for like decades now. In the game, you basically take the role of one out of four vocations, named sorcerers that are damage dealers with lots of AOE; Knights that mostly melee, and that are basically meatshields; Paladins or Pallys that are the all-rounders (really OP the motherfuckers. Healing, Holy Damage, LOTs of Damage and Ranged Damage), finally, there's the Druids, that are basically healers with extra AOE damage for solo-leveling (Think of a Sorcerer with more healings, less damage).

All in all, they're pretty balanced (Taking out Pallys, that shit is broken). With that, you have cities, spells, quests, monsters to kill, and pretty much what every other MMORPG has, though, in some aspects Tibia is a little more difficult, not that it actually matters here.

Now, OTS or Open Tibia Server is like a private custom server some players make on their own using the original game's system, maps (sometimes), and graphics, it's like modded versions of the original game, kinda like Warcraft 3 custom maps or Minecraft heavily modded versions, they are usually easier to play, quicker to gain exp or loot, and add a lot of stuff like new maps, monsters, heck, there's even good pokemon-based OTS out there.

You could actually play hundreds or even thousands of those OTS, each one kind of unique in its own way, some of them add custom spells, some of them simply work with the original Tibia systems. The thing is like I said, you'll level up faster, like REALLY faster here. You could reach LVL 1.000 in hours, or even more, depending on the OTS you're playing.