University of California, Berkeley: Paul Hilfinger's "Type Inference and Unification"
Read slides 1 through 8. The slides are somewhat formal and present a "type calculus." Use these slides to add to your understanding of types.
Click http://web.archive.org/web/20160502145137/http://inst.eecs.berkeley.edu/~cs164/sp11/lectures/lecture22.pdf link to open resource.