Complex sentence: In mathematics, an exotic sphere is a differential manifold "M", such that from a topological point of view "M" is a sphere, but not from the point of view of its differential structure.
Split into simpler sentences: In mathematics, an exotic sphere is a differentiable manifold, "M", that is homeomorphic to the ordinary sphere, but not diffeomorphic. That means that "M" is a sphere from a topological point of view, but not from the point of view of its differential structure.