Availability is a cognitive heuristic in which a decision maker relies upon knowledge that is readily available rather than examine other alternatives or procedures. We make judgments based on what we can remember, rather than based on the complete data.
Representativeness heuristics state that when probability in an unknown event is indiscernible, people find an approximately similar event and approximate probabilities for the unknown event based on the known. This gambler's fallacy overlooks the fact that similarity in one aspect does not correlate to similarity in another aspect. For example, representativeness heuristics would be at work in thinking that driving a loaded pickup truck hauling a car would have a probability of driving speed as the similar experience of driving a loaded pickup truck.
Another practical example of availability heuristic that can be used to demonstrate this concept is the fear that many people have of poisonous spiders. Actually, statistics show that you are more likely to be killed by a champagne cork than a poisonous spider.
Representativeness heuristic is defined as two or more objects being judged to be similar at first glance without further investigation or "looking deeper" (often referred to as stereotyping). In a way, it's a form of prejudice, sort of the "beauty is only skin deep" type of mentality that overlooks what might be in favor of the obvious. An example of this would be to assume that all peoples of Middle Eastern heritage are involved in terrorism.
Availability heuristic, on the other hand, is a methodology that assumes something will happen as easily as we imagine it will. An example of this would be a person being afraid of flying because he thinks the likelihood of being killed in an airplane crash is greater than being killed in an automobile accident. But, this thinking isn't correct in that far more people die on the highways each year than die falling from the sky. But, it seems a much more dramatic way to die and, granted, many more people die at once.