Screencast 18.1
https://www.youtube.com/watch?v=jTINKz-F3fY
In this screencast, Andrew Parsons defines enantiomers in terms of chiral centres, and walks you through the definition of S and R enantiomers.

Screencast 18.2
https://www.youtube.com/watch?v=OzMVQryAoC0
In this screencast, Andrew Parsons defines diastereomers and walks you through examples of diastereoselective reactions.