What's the difference between "Mark for Removal" and "Mark for Complete Removal" in Synaptic Package Manager?

Mark for removal removes the package, but not the configuration files associated with the package. It is equivalent to

apt-get remove package_name

Mark for Complete Removal purges the package, i.e. removes both the package files and its configuration. It is equivalent to

apt-get --purge remove package_name

From the Synaptic Package Manager manual:

3.2. To Remove Packages

Configuration files and user created data (e.g. a website in "/var/www") are not removed from the system by default.

Debian only: You can change the default behavior in the preferences.

Debian only: To remove all files related to the package choose Mark for Complete Removal instead of Mark for Removal.