One of the most prominent themes of this film is revealed on John Keating's (Robin Williams) first day of teaching. He brings his class to the hallway to gaze on the portraits of several dead poets and whispers "Carpe Diem" which is Latin for "seize the day."
This concept is in direct contrast to the opening scene of the film, in which the "four pillars" of the school are recited in an opening assembly: tradition, honor, discipline, excellence. Essentially, the teaching method at Welton Academy has always been one of conformity, recitation, and acceptance of authority. Keating introduces his students to a new way of thinking, which is just that, thinking. He teaches the boys to think for themselves. He teaches them to seize opportunities ("Gather ye rosebuds while ye may...") and to consider the personal consequences over the consequences implemented by authority.
As a result, Keating's students begin to make choices that go beyond the scope of their once very robotic lives. They experience both the positive and the negative consequences of their choices, but most importantly, they learn, grow, and mature.