Oh, wow. It's been a while since I've seen this thread. I actually had someone type me for fun, and he was far more knowledgeable of Jung's original work. ISTJ was what I was typed as, and looking at more explanations solidified it for me. For everyone here, the tests and most resources are bunk, as the tests have an N bias and don't always work with the functions, whereas other resources are generally inept at describing certain functions. Examples include the Si function, where most resources describe it as, "memory, structure, the past." Si is more like the impression of the past within the present moment. Think of two painters painting a scene, and how the art may differ in color, lighting, and other things.
If you want to dive into Jung's original work of the functions, I was recommended Psychological Types. It's free on Google, and chapter X is where you'll find his descriptions. A warning: It's a bit dense, and it's recommended to either read from the start, or perhaps even familiarize yourself with Jung through his other work, like Man and His Symbols.