The idea is similar to Xfire, where users can take screenshots locally and choose to upload some to their online profile. Players have always been able to capture images from many games via in-game screenshot functionality or FRAPS, but the goal of this service is to add a community side to screenshot sharing.
The service supports all games with the Steam overlay with the exception of those utilizing DirectX 8 or lower. You can sign up in Steam settings under available betas.
Read the official news post