I had a basic question regarding Ubuntu's apt-get update/dist-upgrade behavior.  I was unsure of which software packages were updated when apt-get update/dist-upgrade were run.  I read on other SO forums and the Ubuntu site that all the packages listed in /etc/apt/sources.list are updated when the command is run, which makes sense. But, I also read that any package that I installed with apt-get install would be updated too. However, when I look at sources.list, there are only a few Ubuntu repositories listed and none of the many third party software packages I installed are included (like okular, git, or any other package installed manually) in this file.  So does apt-get update actually update all these packages?  If so, how does it know to update all these other packages considering they are not enumerated in /etc/apt/sources.list? Finally, if I wanted to check just one of these programs, say git, for updates manually and install them if there were, how would I go about this?  
Ubuntu software updates with apt-get
linux;ubuntu;apt;upgrade;aptitude