apt-get is the command to do package/application management in Debain based machines such as Ubuntu.
There is a slight difference between update and upgrade options.
Is the command to update the source list, if you modify the source list or you want to make a sync refresh or added new ppa source then you should execute above command.
Command will try to download all the packages which are having updates at apt server and then try to install them if you press “y”. This something like System upgrade to new packages.