we present a review on the most symmetric non-homogeneous manifolds known as cohomogeneity one manifolds, i.e. pseudo-Riemannian $G$-manifolds where $G$ is a closed connected Lie subgroup of the isometry group and the lowest codimension of the orbits under the natural action of $G$ is one. Geometric and topological properties of such manifolds, their orbit spaces and orbit structures will be described and there will be a comparison between Riemannian and non-Riemannian $G$-manifolds to illustrate some basic differences.