The portal is on the right
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
File:Portal Building Blocks.jpg File:Arrow.gif File:Portal.jpg
Media
Here 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.
On 29 October 2010 PCGamer released this video, showing a portal being constructed and used.