What is the definition of a(an) “IT”?

Hi, I’m a high School Student(senior) and I like computers. I’m pursuing for an A+ certificate, MCSE certificate, CNE certificate and more. From this site “TechRepublic” point of view I can see that being a(n) “IT” is very interesting, but I don’t have a clue of what “IT” stands for, means, or do. I’m also very curious of becoming a(n) “IT” professional, when I go to college, I would like to know what courses to take.