The use of the observational properties of the Cosmic Microwave Background radiation (CMB) to constrain the presence of different components in the universe, which perturb the background metric, is nowadays a well-established methodology in Cosmology. In this thesis the methodology is applied to three cases: primordial magnetic fields, a hypothetical electric charge asymmetry in the universe, and the inflaton field. The main part of the thesis deals with the problem of cosmic magnetic fields: assuming that they were formed in the early universe, we derive upper bounds on their intensity and analyse the imprint of a helical magnetic field on the CMB. We also study the effect of a global electric charge asymmetry, and deduce upper limits on the charge of some fundamental particles, partly improving the current ones from laboratory experiments. The first chapters of the thesis are dedicated to the review of the theoretical framework at the basis of the treated topics: we give an overview of the physics of cosmic magnetic fields, from observations to the theory of their generation, of cosmological linear perturbation theory and of the formalism of CMB anisotropies and polarisation.