One’s a sensor and the other’s an intuitor.
…but I imagine you wanted a more substantial response than that.
ISTJ cognitive functions: Si-Te-Fi-Ne
INTJ cognitive functions: Ni-Te-Fi-Se
As you can see, our secondary and tertiary functions are the same, while our primary and auxiliary are flip-flopped from N to S or S to N.
We can both be very clinical types, somewhat reserved. The major difference is, of course, our primary function: Si vs Ni. While the INTJ thinks in relation to the future (‘what is possible if I do this?’), the ISTJ thinks in relation to the past (‘what has happened before when I did this?’). The ISTJ believes in concrete results and trusts prior experience, while the INTJ is more willing to try new and untested approaches if they have promise. The INTJ is innovative where the ISTJ is traditional. The INTJ will also be more interested in the big picture, while the ISTJ will take interest in the details.
A more developed ISTJ can use their Ne to consider those ‘what-ifs’ that come so naturally to INTJs, and developed INTJs can use their Se to see the details that are easily seen by ISTJs.