As an ISTJ, your primary mode of living is focused internally, where you take things in via your five senses in a literal, concrete fashion. Your secondary mode is external, where you deal with things rationally and logically.
ISTJs are quiet and reserved individuals who are interested in security and peaceful living. They have a strongly-felt internal sense of duty, which lends them a serious air and the motivation to follow through on tasks. Organized and methodical in their approach, they can generally succeed at any task which they undertake.
ISTJs are very loyal, faithful, and dependable. They place great importance on honesty and integrity. They are "good citizens" who can be depended on to do the right thing for their families and communities. While they generally take things very seriously, they also usually have an offbeat sense of humor and can be a lot of fun - especially at family or work-related gatherings.
ISTJs tend to believe in laws and traditions, and expect the same from others. They're not comfortable with breaking laws or going against the rules. If they are able to see a good reason for stepping outside of the established mode of doing things, the ISTJ will support that effort. However, ISTJs more often tend to believe that things should be done according to procedures and plans. If an ISTJ has not developed their Intuitive side sufficiently, they may become overly obsessed with structure, and insist on doing everything "by the book".