In which way did Rosa Parks have an impact on American society? Is Rosa Parks the reason for equality in the United States?

The answer to this really depends on whether you subscribe to the “great man” (or, in Parks’s case, “great woman”) view of history.  If you do subscribe to that point of view, you could say that equality exists in the US (to the extent that it actually does) because of her.  If you do not subscribe to this school of thought, Parks is much less important.

The great man view of history holds that important individuals drive historical events.  These individuals cause these events to happen.  In this view, for example, we might say that Hitler caused WWII and that Rosa Parks and Martin Luther King brought about equality in the United States.  People who follow this school of thought would argue that Parks really sparked the Civil Rights Movement by refusing to give up her seat on the bus.  If not for her, the movement would not have begun and we would not have as much racial equality as we do.

To others, Parks herself is not that important in that she did not cause the Civil Rights Movement to happen.  In this view, if Parks had not started the movement, someone else would have.  This is because forces greater than any one individual drive historical events.  In other words, the Civil Rights Movement did not start because Rosa Parks remained seated.  Instead, it occurred because of large historical forces like the legacy of WWII, the Cold War, the increase in American prosperity, and the fact that the rhetoric of American independence emphasizes equality and justice.  It is those larger forces, and not the actions of any single individual, that cause important things to happen.

I tend to subscribe to this second point of view.  Rosa Parks, as an individual, was admirable.  She was willing to fight for what she believed was right.  She helped to begin a movement that made the US a better place.  However, I do not believe we can say that whatever equality we have today exists because of her.  I believe that the Civil Rights Movement would have occurred more or less as it did even if Parks had not been on that bus on December 1, 1955.