There are several different types of unemployment, of varying duration and severity (in terms of implications for the economy).
This PowerPoint goes through the types, and addresses why the Macroeconomic goal is "Low Unemployment," rather than "Zero Unemployment."
This PowerPoint has teacher memos for each slide that can be used as a script when teaching. There are a number of question slides and questions within the memos with possible answers.
If you see any problems please contact me.