  Galactic winds and mass outflows are observed both in nearby starburst
galaxies and in high-redshift star-forming galaxies. In this paper we develop a
simple analytic model to understand the observed superwind phenomenon. Our
model is built upon the model of McKee & Ostriker (1977) for the interstellar
medium. It allows one to predict how properties of a superwind, such as wind
velocity and mass outflow rate, are related to properties of its star-forming
host galaxy, such as size, gas density and star formation rate. The model
predicts a threshold of star formation rate density for the generation of
observable galactic winds. Galaxies with more concentrated star formation
produce superwinds with higher velocities. The predicted mass outflow rates are
comparable to (or slightly larger than) the corresponding star formation rates.
We apply our model to both local starburst galaxies and high-redshift Lyman
break galaxies, and find its predictions to be in good agreement with current
observations. Our model is simple, and so can be easily incorporated into
numerical simulations and semi-analytical models of galaxy formation.
