This is an introduction and a survey on geometric structures modelled on closed orbits of real forms acting on spaces of flags. We focus on 3-manifolds and the flag space of all pairs of a point and a line containing it in $\mathbf P(\mathbf C^3)$. It includes a description of general flag structures which are not necessarily flat and a combinatorial description of flat structures through con- figurations of flags in closed orbits of real forms. We also review volume and Chern-Simons invariants for those structures.