Category theory is important to type theory and semantics (Pierce, see below, also wrote several books on types in programming languages) and offers a different representation of computational flow (monads, arrows).

Benjamin Pierce's "Basic Category Theory for Computer Scientists" (not online) is terse and complete [1].

Wikipedia is good for definitions and there's also a Wikibook on category theory: http://en.wikibooks.org/wiki/Category_theory

There are some brief, informative, and fun lectures on YouTube by the Catsters: http://www.youtube.com/profile_play_list?user=TheCatsters

The natural transformations playlist is where to start, just be ready to use the aforementioned references.

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[1] Natural transformations are the original point of category theory. A text that doesn't cover them is incomplete.