This is a first year, high-school level course on Geometry (which is based on Euclid's elements). It revisits many of the basic geometrical concepts studied in earlier courses, but addresses them with more mathematical rigor. There is strong focus on proving theorems and results from basic postulates.
This video defines angles, shows several examples of angles, and teaches some simple rules for analyzing them using the degrees of a circle, a straight line, and perpendicular lines.