The representativeness heuristic is a rule-of-thumb strategy that estimates the probability of an event based on how typical that event is. While often very useful in everyday life, it can also result in cognitive biases. In causal reasoning, the representativeness heuristic leads to a bias toward the belief that causes and effects will resemble one another.
The difference in the two decision making processes is that one is based upon memory of recalled events while the other is based upon comparison of two events. Availability heuristics is based upon what one can remember; recent or vivid information being more memorable than vague or uncomfortable and thus rejected information. Representativeness heuristics is based on perceived similarities between an unknown and a known event; probabilities for the unknown come to viewed as comparable to the known, though this may not be the true case.
Representiveness heuristic is when objects are considered to be the same based on their outer appearance rather than the reality. A very topical example would be racial profiling, and how assumptions are made about people and their involvement in crime based on ethnicity. Availability heuristic stresses that what we imagine will happen easily. An example of this might actually be getting married. We assume that when we marry, it will be for life, and our imagination blinds us to the reality of high divorce rates.