Nether portals are made by creating a 4x5 (2x3 on inside) door frame out of obsidian and then lighting the inside of the frame on fire with flint and steel or a fire charge (fire charges can be launched at the portal frame with dispensers to activate it, preferably right next to it, allowing automatic ignition). When the portal has been lit a purple mist, as shown in the picture, appears inside it which can be selected by looking at it but not mined, the only way to destroy it is to mine an obsidian block next to it with a diamond pickaxe. However, the mist can be mined in creative mode and, interestingly creates the same noise made when glass is mined. Doing this turns off the nether portal but does not give the player a block of nether portal purple mist. When the player stands inside the portal for the first time, it will create a portal in its equivalent area of The Nether and obviously transport the player to the Nether. A loading screen will appear during the change of worlds. See the Nether Portal page for more information on portal mechanics. Also, trying to get in resource nether, do /spawn Resource_nether.
As of 1.7 Nether portals can be any shape and size!