The term stellar evolution refers to a star's changing stellar structure over its own lifetimes including how long each general phase lasts. The term is generally used to refer to how individual stars live and die rather than to the differences between stars born at different periods, the latter of which might be expected from the term "evolution". Stellar evolution models of main sequence stars take into account the change in the constituents of stars as fusion reduces the hydrogen, i.e., gradually changing stellar structure models. Essentially, they work through stellar structure at a given time, take into account the composition and structure changes-over-time that the workings of the model star imply, work out the resulting structure after some time-step, and repeat.
Such models can be checked somewhat through star counts and the resulting H-R diagrams, to identify the counts of stars in different phases and see if the counts are consistent with the stellar evolution models and what's known of the galaxy's star formation.
Among the general phases in the life of a main sequence star according to well-established models:
Among the factors that have to be taken into account when modeling are the changing composition due to fusion, mass, diffusion, settling due to gravity, mixing/homogenization (e.g., due to convection), the affects of stellar rotation, mass gain and loss (e.g., accretion and stellar wind), interactions among binary stars.