What is the difference between a predator and a parasite?
Good question.

There is no fundamental difference between parasites and predators.

Ecological Interaction

In terms of ecological interaction, they are both defined as an interaction where one species benefits and the other suffers from the interaction. 

Intuition parasite vs predator



In general predation is viewed as a big individual eating a smaller one while parasitism is when a small individual eats a bigger one. These general intuition however fails to consider a number of cases. Here some cases I can think of

Examples when intuition don't match the above picture

Generally considered as predation but does not fit the above picture-based definition:


social animals attacking in groups preys that are much larger than themselves - non-social animal attacking prey larger than themselves
herbivores browsing on large trees


Generally considered as parasitism but does not fit the above picture-based definition:


indirect effect of habitat modification that would rather be considered as parasitism.
Species that take advantage of parental care of another species by mimicking babies (but being larger)
Individuals of large species stealing the habitat built by a smaller species
Batesian mimicry (see this post; Thank you @WYSIWYG and @NL_Derek for the comments)


Population culture vs science literature

In the popular culture, some people call parasites only endoparasites. This concept is also misleading and not so much in accordance with the general literature in biology.