What is the difference of apt-get update and apt-get upgrade

what does “sudo apt-get update” does??

it obtains the information and download the package lists about what updated package versions are available from the source repositories.It’s very important to run this command before installing any packages , and necessary to run it to install the latest updates.

Then what does “sudo apt-get upgrade” does??

It will download the newer versions of packages that are existing in the ubuntu machine , if it knows the newer versions in the mean of “sudo apt-get update”