It stands for Introverted Sensing Thinking Judging. They make up around 10% of the population.
Known as the "Inspectors",they are SUPER dependable and responsible. They usually always follow rules and try to get others to also. If there is a paper due, an ISTJ will have it done 2 weeks before and have it in at the exact time it's due,or earlier. They usually want everyone to follow the rules like they do. So if you ever get snitched on for something it was most likely an ISTJ that told. They have a big respect for authority and won't question it. They tend to be shy and quiet. They are organized and detail-oriented and love it when a job is well-done. They are traditionalists and probably vote Republican. They usually aren't very creative because they are conformists and usually want to spit on everyone that breaks rules. But they can usually succeed at any task because they are methodical and driven. They tend to be a Yes Man.
When stressed out the ISTJ will have meltdowns and go into "CATASTROPHE MODE!". This is when they will picture everything that could go wrong and will beat themselves up for doing something wrong or forgetting to do something then they will depress themselves with all their visions of catastrophe. But all in all ISTJ's can be cool and make loyal friends.
Typical ISTJ- "No,I'm going to have to report you to the police because that is illegal."
Now here is a cooler ISTJ...
Bob-"Hey guys I have some weed if you wanna come over after school."
Cooler ISTJ- "Neh it's cool,but I'll see you tomorrow."