Academics, a subcategory of experts, are people who have studied a particular field at university level for many years, becoming one of a small collection of people who are knowledgeable about one area.
Academics tend to write at length about topics they are interested in (and sometimes even contribute to topics no one cares about). One difference between academics and professors is that academics do not necessarily teach and are not necessarily associated with a particular university.
While most academic professional are highly specialized, they often must be well-versed in other fields that supplement their own. For instance, physicists usually study very complicated mathematics; biologists, psychologists and sociologists need to be fluent in statistical analysis, and theologians who study the Christian religion often study the Greek and Hebrew languages in order to gain understanding of historical texts.
Academics are often accused of (and sometimes guilty of) living in an "ivory tower," in which they interact with a simplified and therefore distorted model of reality as though it were the real world itself (i.e., they "eat the menu," or "mistake the map for the territory") and ignore the complexities and contradictions of the real world, or rest attempts to implement theory-based real-world measures on their assumed authority. This is very seldom an issue in the highly technical "crunchy" sciences, where an idea's validity is generally a relatively clear question and it typically must pass muster based on its applicability and predictive power, say the scientific method. However, this behaviour is more likely in the fuzzier sciences (psychology, religious studies, sociology), and potentially pervasive in politically or emotionally charged areas (e.g. in women's studies on the left end, or economics on the right end of the political spectrum) where the perspective of a highly-invested academic may readily be
twisted informed by emotion, pet theory, or political agenda.