This image (made famous all over the world now as the "End Him Rightly"/Rematch meme) is from the Gladiatoria, a series of dueling manuals from the HRE from sometime in the 15th century. Happy to discuss these in some detail if you want, as they are actually quite interesting in themselves as documents of martial history, but to answer your question directly, the type of shield depicted is called an "écranché."
It was, as depicted here, a small shield primarily used for jousting, thus the declivity in the upper portion, which was made to buttress a cavalier's lance or spear. However, this image is not really to scale; such a shield would be about this size. Or, here is another image of it, being held by someone on foot. As it is a cavalry shield, its place as a shield in hand-to-hand combat is somewhat unusual, but to understand why it's depicted here, we need a bit of a history lesson.
The Gladiatoria, as a combat manual, follows the usual armament structure of a trial by combat (or judicial duel), which had specific weaponry and techniques associated with it. Now, a trial by combat was available only to nobility, and only in certain circumstances -- and it had a specific "menu" of fights that the duelists would engage in, in a set order. This, typically, began with horseback jousting. This is why the écranché is present here.
The odd thing is that the Gladiatoria skips the horseback part entirely, and goes right to techniques for fighting on foot -- yet still using the horseback panoply. Thus, the fellow on the right still using his écranché, except this time it appears to have been affixed to his arm similar to a standard buckler.
He will likely need it, as what is being depicted in this particular panel is one of the "Gladiatoria's" most bizarre techniques -- that of unscrewing one's sword pommel and throwing it at one's opponent as a stunning or distraction technique. The knight on the left, as you can see, is preparing to do that, while the man with the shield is -- hopefully -- preparing to block the throw.
