In this video Doug Jorgesen explains the motivations, basic operations, and some applications of IQ, image reject, and single sideband mixers.

For more information, see the following blog posts:

The Why and When of Mixers for Beginners
Top 7 Ways to Create a Quadrature (90˚) Phase Shift
All About Mixers as Phase Modulators
All About Mixers as Phase Detectors
How to think about IQ mixers
IQ, Image Reject, and Single-Sideband Mixers