Introduction to angles (old)  Angles and intersecting lines  Geometry  Khan Academy  Summary and Q&A
TL;DR
This video covers the basics of angles, including definitions, measurements, and relationships between different angles.
Questions & Answers
Q: How are angles measured?
Angles are measured in degrees or radians. In geometry, degrees are commonly used. A protractor is a tool that can be used to accurately measure angles.
Q: What does it mean for two lines to be perpendicular?
Perpendicular lines are lines that intersect at a 90 degree angle. They are often seen in shapes like squares and rectangles.
Q: How are angles in a circle related?
There are 360 degrees in a circle. For example, if one angle in a circle is 135 degrees, the complementary angle would be 360 degrees  135 degrees = 225 degrees.
Q: What is the difference between complementary and supplementary angles?
Complementary angles add up to 90 degrees, while supplementary angles add up to 180 degrees. These terms describe the relationship between two angles.
Summary & Key Takeaways

An angle is a measure of how wide the intersection is between two lines and is measured in degrees or radians.

Perpendicular lines have a 90 degree angle, while wider angles can be measured using a protractor.

Angles in a circle add up to 360 degrees, with halfway around the circle being 180 degrees and a quarter being 90 degrees.