The server has a custom portals plugin that allows players to create pairs of portals that then allow free travel to the opposite end. To use them, simply stand in one of them and crouch (press shift), and you will be teleported to the opposite portal, wherever that is. Each portal is paired with exactly one other portal, but players are able to create as many portals as they want.
To create a set of portals, first create the obsidian foundation as shown in the picture. This can face either direction. Then in the inside of the portal, place 1 diamond block and 1 lapis lazuli block. If the blocks do not disappear, then the location you are trying to use is already in use by another portal, or the blocks have not been placed properly. Make sure the portal is complete obsidian as shown in the image, including the corners.
Once you've successfully set one end of the portal, you should then travel to where you want the other end to be located. Be careful not to die during this period, if you die before finishing up the 2 portals, the one you've already placed will be lost. When you've reached the other location, simply repeat the process building an identical portal.
The portals allow instant travel, but they can only be placed in the 3 vanilla worlds (the overworld, the nether, and the end.) It is possible to create portal pairs that go from one world to another.
To disable a portal, simply break one of the obsidian blocks. This will permanently destroy both ends of the portal, requiring you to build a new portal at both ends.
All non-player entities within 2 blocks of the portal will be teleported along with you when you teleport.
You can fill the portal in with solid blocks but if you teleport to it from the same dimension you will be stuck in a wall. There is a bug that teleports you to the surface if your filled in portal is in another dimension. This is useful for hiding portals.
Some players choose to use Portal onion routing to increase the security of the portal links.
It is possible to protect your portals from X-ray.
If a pressure plate is placed inside a portal and activated by a player, that player will be teleported as if they had crouched.
The source code for the Portals plugin is available on github.
Player goes to (100, 65, 100) and builds a portal, it is then activated. Player then travels to (-200, 35, 300) without dying and builds another identical portal there. The two portals are now connected, if Player or anybody else stands in the portal at (100, 65, 100) and then crouches, they will be teleported to (-200, 35, 300). If they stand in (-200, 35, 300) and teleport from that portal, they will be teleported to (100, 65, 100).
If Player had died before building the portal at (-200, 35, 300) then the portal at (100, 65, 100) would have been deactivated, and Player would have had to travel back to (100, 65, 100), put in another diamond block and another lapis block, and then start over traveling to (-200, 35, 300). After both portals are active however, Player can die any amount of times and nothing will happen to the portals.
If someone then breaks an obsidian block at either (100, 65, 100) or (-200, 35, 300) then both portals will be deactivated, even if the obsidian block is put back. In this case someone will have to rebuild both portals.