list.save and list.distUpgrade

I am not sure about sources.list.distUpgrade, but the sources.list.save file is a backup of the sources.list file. I know from personal experience (about 5 minutes ago) that the sources.list.save file is extremely important after attempting to add a repository. If software center stops working correctly (EX: closing right after starting) then you need to replace the current sources.list file with the sources.list.save file.

To do this, open a new terminal (CTRL+ALT+T) and type the command

sudo cp /etc/apt/sources.list.save /etc/apt/sources.list

and then

sudo apt-get update

This should fix the corrupt sources.list as long as sources.list.save exists in the /etc/apt/ directory.

Simply put, sources.list.save is just a backup file.