The present Ph.D. thesis consists in a series of experiments carried out in the Laboratory of Quantum Optoelectronics under the direction of professor Benoît Deveaud-Plédran between April 2006 and April 2010. We study the effect of lateral confinement on the dynamics of microcavity polaritons according to two important subjects. On the one hand, we study the polariton relaxation, on the other hand, we study the role of the spin in polariton mutual interactions. We first introduce microcavity polaritons, their optical properties and present the polariton pseudospin formalism. In our sample, polaritons are trapped through their photonic component in cylindrical extensions of the cavity length called mesas. Polariton relaxation is studied in the linear and nonlinear regimes. In the linear regime, we demonstrate that polariton interactions with acoustic phonons are enhanced under lateral confinement. Thermalization, which is forbidden in planar microcavities, is facilitated by confinement, and is very efficient in small diameter mesas. We develop for the first time a comprehensive model of polariton relaxation dynamics under confinement, and highlight the role of polariton states with large exciton content. This work will be profitable to the design of future samples dedicated to the study of Bose-Einstein condensation. In the nonlinear regime, we study the impact of polariton-polariton collisions on the spatial dynamics of microcavity polaritons. In the low-density regime, when the mesa is excited in a coherent superposition of the three lowest energy states, the polariton dynamics is characterized by dipole oscillations. In the high-density regime, we observe a continuous damping of the dipole oscillation. This is due to multiple parametric scattering processes, which redistribute the energy to the benefit of the ground state. Moreover, we demonstrate that the collisional damping is more efficient for collisions between polaritons of the same spin than between polaritons of opposite spins. We investigate in detail the influence of polariton spin in the interactions between polaritons. We first consider the case of planar polaritons. We observe that optical bistability is strongly reduced, or even suppressed, when the system contains polaritons of opposite spins. This results from a pairing of polaritons of opposite spins to form biexciton states. We demonstrate the polarization control of the different optical instability regimes. We then study the case of confined polaritons. As the system is cleaner (we work with a single polariton energy level), we achieve full optical control over the spinor interactions. We demonstrate the first realization of multi-valued switching with a coherent spin ensemble in the solid state. This is a very important step in the research on spin manipulation for the development of spin optoelectronic devices. Finally, we show the influence of the polariton spin on the coupling between planar and confined polaritons and on the control of the polariton fluid dynamics. All our experiments are confirmed by theoretical models. We propose perspectives for the next studies on the dynamics of polaritons and on the coherent spin manipulation in microcavities.