A Portal is a user-created structure that allows a player to travel to The Nether. This is not to be confused with an End Portal, which is naturally found in Strongholds and allows a player to travel to The End. There is talk of there being more portals added into the game in the future (other than The Nether).
To make a portal, the player must first build a 4x5 frame of Obsidian. Once the frame is built, the player should either use Flint and Steel or Fire Charge on one of the Obsidian blocks, Lava will not work. A purple vortex should appear within the frame, and the player can now reach the Nether. To use the portal, simply enter the purple vortex and remain inside for several seconds until the Nether (or the Overworld, if traveling out of the Nether) begins to load. In the 1.7.2 update, Nether Portals can be made into different shapes (any number of Obsidians work now).When building the Obsidian frame, it is actually unnecessary to construct the corners out of Obsidian, or even to include them at all - the portal functions exactly the same without them. Leaving out the corners creates an "economy" version that only requires 10 blocks of Obsidian as opposed to the full 14 that the full frame requires.
When you enter The Nether, a second portal, correlating exactly to the first portal's location in the Nether, is built automatically. It functions identically to the portal the player had made in the regular world, and it is used in the same way to exit the Nether.
It is important to note that a Ghast's fire charge can shut down an active portal. If this happens, you must reactivate the portal yourself as you did in the Overworld. One way to prevent deactivation is by sheltering the portal with cobblestone.Note also that there is only one Nether per world, so all Portals that are constructed in the world will lead to the same Nether. This allows you to use the Nether as an alternate route between two points in the Overworld (e.g. to enter a sealed mine or your house from a location outside). Also note that while Mobs are capable of traveling through a Portal (not allowing you to escape from their pursuit), you have to be prepared for an attack of Zombie Pigmen in the overworld, or one on the nether with overworld mobs.