In the paper we consider time-varying systems with coefficients depending mero-morphically on time. In differential operator representations these systems are described by matrices over a skew polynomial ring with coefficients in the field of real meromorphic functions. Different kinds of indices (controllability, minimal, geometric and dynamical) are introduced and it is proved that they essentially coincide. The input module and the formal transfer matrix are defined and used for an algebraic description of time-varying systems. A characterization of system equivalence is given in these terms and also a complete list of invariants of similarity for time-varying state-space systems.