Weak Squares

Weak Squares-Weak squares can be defined as squares where your opponent can safely post his pieces. Weak squares provide an entry point into your territory and are often the precursor to an attack. Naturally, if you can exploit the weak squares of your opponent, then they will work for you. (Definition- From "Introduction to Chess Strategy for the novice player" by Bill Whited)

Weak Square Lesson - From Exeter Chess Club
Elements of Chess Strategy - Weak and Strong Squares - WikiBooks Article

Lessons from Chess Strategy Blog
Complex of Squares
Complex of Squares 2

Complete Idiots Guide to Chess - Chapter 13 Online is all about Weak Squares

Chess Videos
Chess Strategy - Weak Color Complex in the Endgame - 9 Min Video