If you ever get the chance to read "Please Understand Me II" by David Keirsey, your life will be forever changed. He has the most comprehensive in depth and accurate description of the types I have ever seen. He picked up where Myers Briggs leaves off and completely clarifies the personality types once and for all.
He feels that the S/N difference is the most important difference of all. All the S's are sensing and use concrete language to communicate. They focus on things in the real world. They usually don't have much time for theory as it does not seem immediately useful or practical.
He actually separates all the types into four categories: SP, SJ, NF, and NT. These he refers to as Artisans, Guardians, Idealists and Rationals respectively. Most of the book focuses on the unique traits that these groups have in common. While there are differences between the four subtypes in each group the similarities are remarkable. After reading his book I feel like I can pretty easily define all my friends and family, then once I have them in one of the four groups I can usually figure out the other two with out too much trouble. I/E is pretty easy and for the Sensing side, T/F usually doesn't pose too much of a challenge, but can be a bit more tricky. Then in the Intuitive side you have to distinguish between P and J for each group. That's not usually too hard either. If you know someone for very long, one of the first things you find out is if they are scheduling (J) or probing (P).
Keirsey's theory is also very convincing because it matches up perfectly with the four humors (sanguine, phlegmatic, choleric, and I forget the last one) as well as Plato's version of personalities.
For a long time I thought I was ISTP probably because my dad is ISTP and some the traits rubbed off like his sense of adventure and distrust of authority. According strictly to Myers Briggs it was hard to really figure out which one I was since she has the two as being very similar. But Keirsey has them in very separate groups, Artisans and Rationals even though they are just one letter off. Because it is the S/N relationship, it is the most important even though maybe not the most noticeable.
Anyways, "Please Understand Me II": Great book I may just buy hundreds of copies and drop them from a hot air balloon onto the masses.