Utkata Konasana or Goddess Pose. And is this not the best illustration ever?! Rob Osborne, you rock. I've included some more examples of his brilliance at the bottom of this post.
Goddess pose is challenging. If you have any ballet experience, it's basically a demi plie. If you're one of those people who struggled with turnout in ballet (like me), you'll find this pose especially challenging.
Here is a pretty good rundown on the pose. And here is a great little flow between goddess and another pose...so simple and not much movement, but so hard! [youtube http://www.youtube.com/watch?v=02feK4NG46s?feature=player_detailpage&w=640&h=360]
My biggest tip for this pose is to pick up your toes! It will help you connect to your legs. Goddess is excellent for strengthening your outer hips and brings a lot of heat (thigh burnage) in the hips which make it easier to work on flexibility. So if you're interested in deepening your hip flexibility, start with this pose before stretching in things like Pigeon of Lotus.