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.

Once the frame is constructed, the player then sets the space inside the frame on Fire.

This creates a swirly purple vortex inside the frame. When the player steps through, they are taken to Hell



Media
No gameplay footage has been released yet, but there are some preparation videos and commentaries in which people craft portals according to notch's specifications. here and here.

On 10/10/10 Notch revealed the above screenshot on his blog showing portals.