The Skrull are a fictional species of aliens that appear in the Marvel universe, mainly in Fantastic Four and Avengers

A Skrull

comics.  The Chitauri are counterparts of them from the Ultimate Marvel universe and both the Skrulls and Chitauri can shapeshift.  


Skrulls are usually seen with having green skin and pointy ears and wear a black jumpsuit which covers most of their body except their face.   They look similar to the Green Goblin.