Mirrorless lenses are designed for use only on mirrorless camera bodies. Because mirrorless bodies do not have a mirror box inside them (hence the name), the lenses are designed in a different way. The result is a shortened focal flange distance, or distance between the lens mount and the plane of the camera sensor. Placing the lens closer to the camera sensor has several advantages for your overall image quality, and using these mirrorless lenses, which are native to your camera body, will produce faster response times and better image quality than an adapted SLR lens.
Just as with SLR lenses, some manufacturers have mirrorless lenses that are designed specifically for smaller framed bodies, and some that are designed for full frame bodies. With mirrorless lenses, however, using a lens from a different sensor size can sometimes require the use of an adapter.