Answer:
Upgrade refers to upgrading to a new version from old version
Clean Install: Removing old version completely and install from scratch
Explanation:
Let you want to upgrade windows server 2008 to windows server 2016.There are 2 options
1. you can directly migrate windows server 2008 to 2016 by keeping everything as it is , so all the data and applications on the system remains as it is and only OS will upgrade.
2. uninstall everything and clean install windows server 2016.Here all previous applications and data are erased
In the above case if all applications on the system are compatible with windows server 2016 from 2008 we can go for upgrade directly otherwise it is better to go with clean install as we need different version of application which is compatible with windows server 2016