One of the very distinct features of Xenomorphs, is that they can climb on any solid surface, for example, upside down, or sideways. They also have dorsal tubes on they're backs, which are used as breathing instruments. Xeno's do not require the same elements as us, for example, they don't need the same amount of oxygen as us, if any is needed at all, and this allows them to stay underwater, or in space, for very long durations without dying. Another characteristic that stands out is their acidic blood. It has a green marsh color to it, and can melt through steel, stone, flesh and bone, etc. The final characteristic that I think stands out more than all the others, is the Xenomorphs second pair of jaws. They are very, VERY powerful, and can punch through skulls without any trouble. And plus, they're really creepy.