The Myr are artifact creatures native to the world of Mirrodin, little metal automatons who carry out tasks. Originally, they were the eyes and ears of Mirrodin's planar regent, Memnarch. Myr are extremely specializable to their intended task; most are tiny errand-runners, but their basic design can be adapted to war, spying, or even magic.
Most of the myr are dedicated servitors. Since the defeat of Memnarch, they have gained new purposes, devoting their metallic little lives to doing what needs doing. That's the essence of being a myr: doing the duty it was built for. Some of them channel mana from the lands and suns of Mirrodin. Some of them rouse and empower their fellow myr.
Myr vary in appearance just as they do in function, but they tend to have a characteristic myr head shape, that swooping "beak," jointed limbs, and smooth metallic complexion. It's the trademark design of modest Mirran industriousness. Myr are made by Myrsmiths
Myr reservoirs appear all across the metal surface of Mirrodin. Each reservoir looks like an artificial crater, its high walls flared up around a metallic opening that leads down into the depths of the plane. The aperture provides occasional flares of mana, not unlike the five huge lacunae from which the five mana-suns emerged. Unlike the lacunae, though, the mana is distinctly myr-oriented, flavored with the tang of metal. Their ebbing mana-flows are naturally suited to the creation of myr-like designs.
Each reservoir is surrounded with a kind of accretion disk of defunct myr husks. Although the reservoir doesn't itself bring new myr into the world, it is fertile with the raw materials for the construction of myr, and is often used by artificers and industrious myr alike to build myr legions.