The lithosphere is the rigid outermost layer of the Earth, composed of the crust and the uppermost part of the mantle. It's broken into large tectonic plates that move slowly across the Earth's surface, driven by the convection currents in the mantle.
The lithosphere is not literally floating on the mantle, but rather it rests on the asthenosphere, a softer, more viscous layer of the upper mantle. The lithospheric plates can move across the asthenosphere because the asthenosphere is relatively weak and deformable.
