Interior Angles of a Triangle: the angles on the inside of a triangle; always add up to 180 degrees
Exterior Angles of a Triangle: the angle between one side of a triangle and an extension of the adjacent side
Source: http://www.mathopenref.com
This video gives a visual of how the interior angles of a triangle relate to one another (by adding up to 180 degrees), and gives a few examples with variables toward the end.
This video explains exterior angles and their relationship to the interior angles of a triangle (also called the exterior angle theorem).
