Nether portal

The Portal is part of the October 31st Halloween Update. It consists of a frame of Obsidian, four blocks wide, by five blocks tall. The corners of the frame are not required, and only serve for aesthetic purposes. Once the frame is constructed, the player then sets the space inside the frame on fire, using a flint and steel. This creates 6 portal blocks inside the frame, resembling a vortex. Here is a diagram of the process:

When the player stands in a portal block for a few seconds, the player is taken to The Nether. The player can step out of a portal before it completes its animation to abort the teleport. A portal will also be created in the place where you enter The Nether, which you can again enter to be returned to the normal world. The purple portal blocks emit light, so passive mobs may crowd around active portals at night. Like Bedrock, portal blocks cannot be broken (although they can disappear), however they can be placed with modding.

When a portal is first created from the overworld to the Nether, there is no guarantee that using the reverse portal will send you back to the original portal. The reason for this is unknown, but is possibly related to placement issues (the Nether portal might have to be moved because it would otherwise be in solid rock/lava/etc.)

It is possible to make a portal you build on the overworld go both ways with a portal in The Nether. To do so, a new portal must be built in The Nether in a position relative to where the portal you first built is. When you travel to The Nether, and then return, you usually emerge from a different new portal somewhere near your original one. Work out the relative position of the new portal and your original, and remember that travel in hell is around 1 block = 8 blocks on Earth. This means that, at some locations on Earth, that a new portal will NOT be created, as the original one is in the "correct" spot. Build a new portal in The Nether at the appropriate location, and your original portal should now link to it both ways.

Portals can deactivate if there is fire adjacent to it (started by the player or Ghasts). TNT can also disable a portal, but cannot destroy the surrounding Obsidian. It is possible to "re-ignite" portals by setting the space inside the frame on fire once again.

It has been confirmed that portals do not work in multiplayer yet; while they can be created, they can not not teleport you. Server settings can be changed in order to visit The Nether in multiplayer.

Notch has mentioned it is extremely unlikely, but possible, for Ghasts to appear on Earth near a portal.

Trivia

 * It is interesting to note that if you place two portals on top of one another you will come out of the bottom one on Earth and the top one in The Nether.
 * It has been reported that building multiple portals on Earth within a certain proximity will all lead to the same portal in The Nether, and vice versa.
 * If a person somehow runs out of flint and steel and disables all of his portals, it is still possible to relight the portal by having a ghast fire at you and hit the portal instead of you. Though this is difficult, it is possible.
 * When you look through an activated portal where there's water behind it, the water will not be visible.
 * If there is no room on one side of the portal you built in the real world (i.e. against the side of a mountain), you might not return to it when using the same portal on the Nether side. Instead, you will arrive at a newly spawned secondary portal in the real world, in the same direction but some distance from the original portal, where there is sufficient room (i.e. within a cave). It is possible to use this to create an Obsidian farm by mining the secondary portal and then traveling through the primary portal again, repeating until the desired amount of Obsidian is obtained.
 * If spawned through modding, portal blocks will automatically disappear after a few seconds, or when a block is placed adjacent do it. The only exception is to place an obsidian block, and a portal piece next to that. The portal piece will disappear once the obsidian block is gone.
 * Even when a portal is built with only 10 blocks of Obsidian (by leaving out the corners), the portal frame spawned on the other side will have the full 14 blocks.
 * If a portal created in The Nether would exit into midair on Earth, the overworld side will have four additional blocks creating a larger ledge to stand on.
 * It may be a glitch, but the player can strike portal blocks with tools, fists, and blocks, but objects will pass through them. This also explains why you can't put blocks and fire inside the portal frame while activated.
 * WARNING! Riding a minecart into a portal has some adverse effects. The player can go through the portal, but remains stuck in the minecart, within the portal's swirl effect. Reloading that saved world again will drop the player into the Nether, possibly without a matching Nether-portal. Riding a pig through a portal also gives this same effect.
 * It is also possible to create a portal by building a frame out of glass, filling it with lava blocks (either with buckets or inventory hacking), and placing a bucket of water in one of the lava blocks. Whether this is a glitch or an intentional decision is unknown.

Media

 * Here and here are two preparation videos with commentary, in which people craft portals according to Notch's specifications.
 * On 10 October 2010, Notch revealed the above screenshot on his blog showing portals.
 * On 29 October 2010 PC Gamer released this video, showing a portal being constructed and used.