The server operates on its own custom server software that allows for hundreds of players at once on the same world. It has taken significant time and effort to develop and you may read more about it here.
The server is based on the survival gamemode, set to the hard difficulty. There are however many changes to improve server performance and gameplay. These are listed below.

World generation
The world uses a modified vanilla terrain generator, to prevent players from generating their own copy of the world.

Phantoms and the phantom mechanic has been disabled entirely.

Monster spawning is reworked, the server will attempt to spawn more monsters more closer to the player.

Player spawning
Players spawn within a distance of 2048 blocks from the 0, 0 center of the world. The spawn mechanic has been changed to allow for more variety for where a player spawns.

Players are allowed to use cheats and modifications in the server. However, the server may not allow full functionality of your cheats. This is mainly to prevent harmful movement cheats.

Item duplication
There is a system in place to attempt to prevent duplication of items.
This is simply an enhancement added to further prevent these and to patch them faster. Duplication is unavoidable in Minecraft and inevitable to occur. Duplicating is still allowed while possible. Duplicated items are not removed.

Maps generate with a random and hidden ID. Players will see a random ID number displayed on their map.

The server will hide information about a players health and equipped items status, so that this can no longer be retrieved by cheats.

Nether dimension roof and voids
These have been blocked from being accessed and void areas will deal more damage to players.

The chat has a few additions that can be viewed here.

Death messages
The server has customized death message texts.

Health and hunger
Health and hunger has been slowed down slightly, with the regeneration of health lingering for longer when below a full food bar.

Ender pearls
Thrown ender pearls do not vanish when the player dies within their render range.