From the Stanford Encyclopedia of Philosophy > http://plato.stanford.edu/entries/category-theory/

Category theory is a general mathematical theory of structures and sytems of structures. It allows us to see, among other things, how structures of different kinds are related to one another as well as the universal components of a family of structures of a given kind. The theory is philosophically relevant in more than one way. For one thing, it is considered by many as being an alternative to set theory as a foundation for mathematics. Furthermore, it can be thought of as constituting a theory of concepts. Finally, it sheds a new light on many traditional philosophical questions, for instance on the nature of reference and truth.

John Baez on Categories, Quantization, and Much More. > http://math.ucr.edu/home/baez/categories.html

• “Basic Category Theory for Computer Scientists”, Benjamin C. Pierce
• “A Categorical Manifesto” by Goguen http://citeseer.nj.nec.com/goguen91categorical.html
• course notes > http://www.let.uu.nl/esslli/Courses/barr-wells.html
• several indtroductory overviews via. good math, bad math > http://scienceblogs.com/goodmath/goodmath/category_theory/