It is known that the number of ground states is very big. This landscape of string vacua contains Calabi-Yau manifolds with vector bundles, D-brane orientifolds, flux vacua and also non-geometrical backgrounds. We will review some of their mathematical and physical properties, like T-duality, group structures and also some statistical properties. At the end of the talk we are planning to discuss transitions between different string vacua by domain walls.