What is NROTC........?

I really want to become a Marine officer, and the US Naval Academy is real hard to get into. So I heard of this NROTC thing, what is it. Will it give me the same oppurtunity as the USNA will? I want to be an Officer in infantry for the United States Marines, and if our country goes to war I dont want to be sitting on the sidelines watching while I could be making a difference.