It's not astrology, it's typology based on Carl Jung's contributions to psychology.
Basically that every person belongs to a "type", a certain mode of functioning through which he/she gains energy. These types are randomly distributed, irrespective of upbringing, area and gender and remain the same from birth.
I would simply just redirect you to MBTI, but MBTI is incredibly flawed. So instead of the whole system (which is just loose ideas floating around on internet forums) I'll just redirect you to a description of the type you probably belong to:
http://personalitycafe.com/intj-articles/10990-lenore-thomsons-intj-type-description.html
EDIT: wait, here's something more entry level:
http://www.personalitypage.com/INTJ.html
The first link uses terminology you're probably not familiar with. Not a good place to start!