Fetches the list of available updates. It downloads package lists from the repositories and “updates” them to get information on the newest versions of packages and their dependencies. It will do this for all repositories and Personal Package Archives
sudo apt-get update
Upgrade is used to install the newest versions of all packages currently installed on the system from the sources enumerated in /etc/apt/sources.list.
sudo apt-get upgrade
Dist-upgrade in addition to performing the function of upgrade, also intelligently handles changing dependencies with new versions of packages; apt-get has a “smart” conflict resolution system, and it will attempt to upgrade the most important packages at the expense of less important ones if necessary.
sudo apt-get dist-upgrade