C. G. Jung's book Psychological Types has had a great influence on everyone interested in classifying people according to their personality types. Jung distinguished two kinds of "attitudes," introversion (I) and extraversion (E), and six kinds of conscious functions, thinking (T) vs. feeling (F), intuition (N) vs. sensing (S), and judging (J) vs. perceiving (P). Therefore, according to Jung, there are sixteen basic personality types that combine these measures: for example, INTJ, or ESFP. It is of course much more complicated than that, but those are the sixteen main types he identifies. His book also surveys the efforts of philosophers and psychologists to classify personality types going all the way back to the ancient Greeks.
Jung's Psychological Types is covered by eNotes. You can access the website by clicking onto the links below.
You might find it interesting to test your own personality type through a Briggs-Meyers personality test, which could find online.