Two distinctive features of Gamma Ray Bursts are that they are relativistic sources and that they have a jet. I will present what can be understood on their dynamic and geometry from currently available observations. The estimate of the bulk Lorentz factor in GRBs open the path to a unifying interpretation of the "color-energy" relations of these sources. A link in GRB jets between bulk Lorentz factor and the jet opening angle (the faster - the narrower, respectively) is also predicted. Through a population synthesis code it is possible to test this hypothesis and discuss several implications on the (prompt and afterglow) emission properties of GRBs.