ISFJs are characterized above all by their desire to serve others, their "need to be needed." In extreme cases, this need is so strong that standard give-and-take relationships are deeply unsatisfying to them; however, most ISFJs find more than enough with which to occupy themselves within the framework of a normal life. (Since ISFJs, like all SJs, are very much bound by the prevailing social conventions, their form of "service" is likely to exclude any elements of moral or political controversy; they specialize in the local, the personal, and the practical.)
ISFJs are often unappreciated, at work, home, and play. Ironically, because they prove over and over that they can be relied on for their loyalty and unstinting, high-quality work, those around them often take them for granted-- even take advantage of them. Admittedly, the problem is sometimes aggravated by the ISFJs themselves; for instance, they are notoriously bad at delegating ("If you want it done right, do it yourself"). And although they're hurt by being treated like door mats, they are often unwilling to toot their own horns about their accomplishments because they feel that although they deserve more credit than they're getting, it's somehow wrong to want any sort of reward for doing work (which is supposed to be a virtue in itself). (And as low-profile Is, their actions don't call attention to themselves as with charismatic Es.) Because of all of this, ISFJs are often overworked, and as a result may suffer from psychosomatic illnesses.
In the workplace, ISFJs are methodical and accurate workers, often with very good memories and unexpected analytic abilities; they are also good with people in small-group or one-on-one situations because of their patient and genuinely sympathetic approach to dealing with others. ISFJs make pleasant and reliable co-workers and exemplary employees, but tend to be harried and uncomfortable in supervisory roles. They are capable of forming strong loyalties, but these are personal rather than institutional loyalties; if someone they've bonded with in this way leaves the company, the ISFJ will leave with them, if given the option. Traditional careers for an ISFJ include: teaching, social work, most religious work, nursing, medicine (general practice only), clerical and and secretarial work of any kind, and some kinds of administrative careers.
While their work ethic is high on the ISFJ priority list, their families are the centers of their lives. ISFJs are extremely warm and demonstrative within the family circle--and often possessive of their loved ones, as well. When these include Es who want to socialize with the rest of the world, or self-contained ITs, the ISFJ must learn to adjust to these behaviors and not interpret them as rejection. Being SJs, they place a strong emphasis on conventional behavior (although, unlike STJs, they are usually as concerned with being "nice" as with strict propriety); if any of their nearest and dearest depart from the straight-and-narrow, it causes the ISFJ major embarrassment: the closer the relationship and the more public the act, the more intense the embarrassment (a fact which many of their teenage children take gleeful advantage of). Over time, however, ISFJs usually mellow, and learn to regard the culprits as harmless eccentrics :-). Needless to say, ISFJs take infinite trouble over meals, gifts, celebrations, etc., for their loved ones--although strong Js may tend to focus more on what the recipient should want rather than what they do want.
Like most Is, ISFJs have a few, close friends. They are extremely loyal to these, and are ready to provide emotional and practical support at a moment's notice. (However, like most Fs they hate confrontation; if you get into a fight, don't expect them to jump in after you. You can count on them, however, run and get the nearest authority figure.) Unlike with EPs, the older the friendship is, the more an ISFJ will value it. One ISFJ trait that is easily misunderstood by those who haven't known them long is that they are often unable to either hide or articulate any distress they may be feeling. For instance, an ISFJ child may be reproved for "sulking," the actual cause of which is a combination of physical illness plus misguided "good manners." An adult ISFJ may drive a (later ashamed) friend or SO into a fit of temper over the ISFJ's unexplained moodiness, only afterwards to explain about a death in the family they "didn't want to burden anyone with." Those close to ISFJs should learn to watch for the warning signs in these situations and take the initiative themselves to uncover the problem.