Terraria is a side scrolling RPG styled platformer that was released on May 16, 2011. The main features of Terraria is the ability to actively edit the world through the use of tools and placing items that you craft or harvest and skill based combat against many sorts of enemies. The game is 2D rather than 3D. It is very much like Minecraft, in both style, objective and slimes.
You can find more information about the game on the Terraria Wiki.
Simon and Lewis did a short 3-part preview of Terraria before it was officially released, where they explored a small amount of the games content.
Simon recorded a "Simon Plays!" video with several Yogscast members including Duncan and Rythian to celebrate the game's official release on May 16.
Simon and Lewis later Livestreamed Terraria in December with several members of the famous YouTube group "The Creatures". They were joined by Pirate Tinman, who made an appearance in the Skype chat too. Any footage the Yogscast has on this Livestream has not been released, however footage can be found on YouTube, released by Creatures members.
Two other members of the Yogscast, Sips and Sjin, have begun a "Let's Play" on their channels. The first episode aired on Sips' Channel (www.youtube.com/user/YogscastSips) on the 23rd of March 2012.