apt-get update && apt-get upgrade

I noticed that at times when I am doing my usual updating to my system from the terminal, I will get a message:

    The following packages have been kept back:

Doing the usual:

    apt-get update && apt-get upgrade

will update / upgrade the packages that are  installed.  If new packages have changed "dependencies", then they are held back.  In other words, the upgrades may depend on packages that are new to your system and since they aren't available they will be held back.  To update the packages to the newer versions and fix the dependencies issue, you can run:

    apt-get dist-upgrade

Now the new packages are acquired that satisfy the dependencies of the upgrades, and your system will then update with the newer packages.