Nether Portal

A Nether Portal is a doorway used to travel between the normal world and the Nether. They consist of an obsidian frame, with a 2x3 space in the middle.

Building a Portal

A portal requires 10 or 14 blocks of obsidian to build.

Portals generated by the game have filled-in corners, but lazy miners often leave them out to save on materials.

You will need a diamond pickaxe to mine obsidian.

Once the portal is built, it needs to be set on fire with a Flint and Steel to activate it.

Larger Portals

The smallest size of portal is a 4x5 frame as described above. It is possible, however, to build portals at larger sizes, up to 23x23.

Relighting your Portal if it goes out

If you are in the Nether, be aware that the explosion from a Ghat's fireball can put your portal out. It's a good idea to build a house around your Portal to stop this from happening.

If it does happen, there are several ways to relight your Portal and get home.

Flint and Steel

If you brought one with you, that's the easy way!

Ghast Fireball

If your Nether Portal is hit for a second time by a Ghast, it will relight it, so try and get a Ghast to attack you while you're standing by your Portal.

Fire Charge

All the components to make a Fire Charge can be found in the Nether.

Source: http://www.minecraft101.net