A Great Circle is any line of Longditude, and, the Equator. A Small Circle is any line of Latitude (excluding the Equator).
Don't get confused by straight lines on a chart... dependant upon the Chart Projection a straight line could be anything... a Rhumb Line or a great Circle Track.
Good luck... it's a less than easy science to understand.
In simplistic terms, stretch a piece of string between any two points on a football... that's a Great Circle! If you were to have a piece of string (tied from end to end) that was any less (in length) than the circumference of the football and that you laid it onto the surface of the football... that's a Small Circle.
Cheers
TCF