We often want the judgment of "experts" as we make important decisions in life, health, and business. But what exactly is an expert?
One aspect of the idea is the possession of a large fund of specialized knowledge. A civil engineer specializing in bridge design is an expert in part because he or she has mastered the mathematics and physics describing the mechanics of large physical systems, the chemistry and properties of various materials, and the best practices that currently exist for ensuring safety. So the expert can be relied upon to have command of the most up-to-date and scientific specialized knowledge about a topic area.
A second aspect of what we have in mind from an expert is a person who has had significant experience in acting and judging in a particular context (and who has refined his/her behavior according to the outcomes of those experiences). An experienced pilot has had the experience of landing an Airbus 330 at O'Hare Airport many times in many different weather conditions. As a result, he/she is ready to respond quickly to unusual wind conditions in a subsequent landing. Essentially this aspect of expertise comes down to trained behavioral skills and responses, permitting the expert to quickly adapt to somewhat unusual circumstances in the future. This is a combination of "muscle memory" and refined habit; it is the trained sensibility and physical awareness of an expert player.
Another aspect of expertise is refined sensibility and perception. This is more akin to the trained palette of a wine connoisseur -- a refined ability to discern the flavors and odors of a pinot noir that are perceived more generically by the occasional wine drinker. Here the idea is that there are aspects of the world around us that can only be perceived and discerned by a trained observer. Michael Polanyi describes this aspect of medical training in Personal Knowledge: Towards a Post-Critical Philosophy (link).
A skilled physician has elements of all three kinds of expertise: the background fund of knowledge of physiology and disease that comes from medical training, the body of experience from which he/she has learned to judge and act, and a refined sensitivity to the minor indicators of disease that a non-expert would likely overlook. A skilled physician can diagnose an illness in a patient based on a body of symptoms, and he/she can prescribe a course of treatment that is likely to lead to alleviation of the disease or its symptoms.
An experienced news reporter may have an unusual ability to "see" the story that underlies a few unusual activities by the city planning commission, and he/she may have a skilled ability to summarize the facts of a story in a logical and readable way. The former is a developed skill of reading a situation; the latter is a developed skill in presenting a sequence of statements in a logical way. Both of these abilities are skills the reporter has cultivated through prior experiences and the feedback that came from success and failure. Further, the reporter may be an expert concerning the facts of a story -- he/she may understand real estate law, the business of real estate development, and the forms of collusion that are common in major development projects.
The reason we seek out the advice of an expert in any of these senses when we are confronted with a high-stakes decision is that we think the expert has a better basis for understanding the problem we face and the options that are available to us, and a better basis for weighing the options. We think the expert can help us make the most reliable judgments and decisions in difficult cases. So appeal to an expert is a strategy for us to reach either knowledge or strategy that is better aligned with the world than our own intuitions and analysis would permit.
As we consider the role of experts in helping us reach personal and collective decisions, it is important to keep track of the epistemic warrant of various kinds of expert judgments. When we consider how best to respond to a nuclear power plant accident like Fukushima, we want nuclear engineers as experts who can advise the best short and medium-term remedies to the immediate hazards. But we might be more skeptical about the judgments of these same experts when it comes to assessing the long-term viability of nuclear power. The assumptions on the basis of which the expert offers the latter kind of judgment may actually be very shaky, whereas the knowledge of nuclear design and physics that is the basis of the former kind of judgment is very strong.
There is a place within the sociology of science and the sociology of knowledge for study of "expertise", and some sociologists like Marion Fourcade have turned their attention in this direction. Expertise can be viewed as a kind of social capital, on the basis of which the expert and those who employ his/her services are able to gain various kinds of advantages. Fourcade's Economists and Societies: Discipline and Profession in the United States, Britain, and France, 1890s to 1990s is not specifically focused on the social composition of expertise, but it offers a good example of how the question might be pursued.