ISTJs in short:

Serious, quiet, earn success by concentration and thoroughness.
Practical, orderly, matter-of-fact, logical, realistic, and
dependable.  See to it that everything is well organized.  Take
responsibility.  Make up their own minds as to what should be
accomplished and work toward it steadily, regardless of protests or
distractions.

About 6% of the population.

Details about ISTJs:

ISTJs are characterized by decisiveness in practical affairs, are
the guardians of time-honored institutions, and, if only one
adjective could be selected, 'dependable' would best describe this
type which represents about 6 percent of the general population.
The word of ISTJs is their bond, and they experience great
uneasiness by thoughts of a bankrupt nation, state, institution, or
family.

Whether at home or at work, this type is rather quiet and serious.
ISTJs are extraordinary persevering and dependable.  The thought of
dishonoring a contract would appall a person of this type.  When
they give their word, they give their honor.

ISTJs can be counted on to conserve the resources of the
institution they serve and bring to their work a practical point of
view.  They perform their duties without flourish or fanfare;
therefore, the dedication they bring to their work can go unnoticed
and unappreciated.

ISTJ's interest in thoroughness, details, justice, practical
procedures, and smooth flow of personnel and material leads this
type to occupations where these preferences are useful.

For example, ISTJs make excellent bank examiners, auditors,
accountants, or tax examiners.  Investments in securities are
likely to interest this type, particularly investments in blue chip
securities, ISTJs are not likely to take chances with their own or
others' money.

ISTJs can handle difficult, detailed figures and make sense of
them.  They communicate a message of reliability and stability,
which often makes them excellent supervisors of, for example, a
ward of a hospital, a library, or a business operation.

They would be capable of handling the duties of a mortician, a
legal secretary, or a law researcher.  High school teachers of
business, home economics, physical education, and the physical
sciences, are ISTJs, as are top-ranking officers of the Women's
Army Corps.  Often this type seem to have ice in their veins, for
people fail to see an ISTJ's vulnerability to criticism.

ISTJs are patient with their work and with procedures within an
institution, although not always patient with the individual goals
of people in that institution.  ISTJs will see to it that resources
are delivered when and where they are supposed to be; materiel will
be in the right place at the right time.  And ISTJs would prefer
that this be the case with people, too.

As a husband or wife, the ISTJ is a pillar of strength.  Just as
this type honors business contracts, so do they honor the marriage
contract.  Loyal and faithful mates, they take responsibilities to
children and mate seriously, giving lifelong commitment to these.
'Duty' is a word the ISTJ understands.

The male ISTJ sees himself as the breadwinner of a family, although
he can accept a working wife - as long as responsibilities to
children are not shirked.  The male ISTJ's concept of masculinity
is patriarchal, and both female and male ISTJs make steady,
dependable partners.  The female ISTJ may abandon the frivolous for
the sensible and may not always deepen her sensuality.

As parents, ISTJs are consistent in handling children, and the
rules of family are made clear.  A rebellious, nonconformist child
may have a difficult time, however, with an ISTJ parent - and vice
versa.  As a child, the ISTJ is apt to be obedient and a source of
pleasure to parents and teachers.

Although ISTJs are outstandingly practical and sensible, they can
marry people who are thoroughly irresponsible, with the marriage
developing into a relationship more parent-to-child than
adult-to-adult.

The ISTJ fluctuates from being rescuer to reformer of the wayward
mate.  The marriage then becomes a lifelong game: On one side,
there is Irresponsibility, Promise of Reform, Brief Period of
Reform, and Irresponsibility again;  on the ISTJ's part, the cycle
is Disapproval, Rescue, Scolding, Forgiveness, Acceptance of
Promise To Do Better, and on and on.

This pattern often is seen when an ISTJ marries an alcoholic and
enters a life of care taking punctuated by periods of anger and
rejection.  Somehow, although ISTJs can accept periodic fickleness
and selfishness in significant others, they do not see this kind of
behavior as acceptable in themselves.

ISTJs have a distaste for and distrust of fanciness in speech,
dress, or home.  The ostentatious is abhorred, and a neat, orderly,
and functional home and work environment is preferred.  Durability
of furnishings are of primary concern, esthetics given slim
consideration.  The clothes of an ISTJ tend to be practical and
durable rather than in the latest style or luxurious.  "No
nonsense" in both food and clothes seem characteristic of this type
who tend not to be attracted by exotic foods, beverages, or places.

The male ISTJ may enjoy stag, men-only parties and use a different
sort of language when only men are present.  The yearly hunting or
fishing trip as a male ritual is often a part of recreation for an
ISTJ.  More than the female, the ISTJ male is apt to be involved in
community service organizations that transmit traditional values to
the young, such as Boy Scouting.  They understand and appreciate
the contributions these groups make in preserving the national
heritage.

Along with the SJs, the ISTJ takes particular delight in festive
occasions held in the context of rituals, for example, weddings,
holiday feasts, and birthdays.  At work, the ISTJ is apt to see the
holiday office party as a necessary nuisance and would be likely to
participate and enjoy these events.