The portal is on the right
The Portal is part of the October 31st/Halloween Update.
It consists of a circle of Obsidian, four blocks wide, by five blocks tall.
Once the arch is constructed, the player then sets the space inside the arch on Fire.
This creates a swirly purple vortex inside the arch. When the player steps through, they are taken to Hell
File:Portal Building Blocks.jpg becomes File:Portal.jpg
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.