Myr are the artificial constructs built by Memnarch that he used to observe the surroundings and inhabitants of Mirrodin. The myr, in form and function, are modeled after ants, as is evidenced by their appearance in card art and that "myrmex" in ancient Greek means "ant."
Myr are almost only present in Mirrodin block (with the exception of Sarcomite Myr in Future Sight), and all are artifact creatures. They embody the philosphy of Mirrodin block in that most myr cards support artifacts in one way or another. In this way, the myrs are actually a highly supported tribe, considering that all of them are artifacts, themselves.