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.