  An analytic model of long string network evolution, recently developed by the
authors, is presented in detail, and modified to describe string loop
evolution. By treating the average string velocity, as well as the
characteristic lengthscale, as dynamical variables, one can include the effects
of frictional forces on the evolution of the network. This generalized
`one-scale' model provides a quantitative picture of the complete evolution of
a string network, including the prediction of previously unknown transient
scaling regimes and a detailed analysis of the evolution of the loop
population. The evolution of all cosmologically interesting string networks is
studied in detail, and possible consequences of our results are discussed.
