University of California, Berkeley: Paul Hilfinger's "Type Inference and Unification"
Study slides 8 through 19. The slides supplement the readings above with type examples. A binding is a substitution of a type expression for a type variable.
Click http://web.archive.org/web/20160502145137/http://inst.eecs.berkeley.edu/~cs164/sp11/lectures/lecture22.pdf link to open resource.