What does "sudo apt-get update" do?

In a nutshell, apt-get update doesn't actually install new versions of software. Instead, it updates the package lists for upgrades for packages that need upgrading, as well as new packages that have just come to the repositories.

• apt-get update downloads the 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 PPAs. From http://linux.die.net/man/8/apt-get:

  Used to re-synchronize the package index files from their sources. The indexes of available packages are fetched from the location(s) specified in /etc/apt/sources.list(5). An update should always be performed before an upgrade or dist-upgrade.

• apt-get upgrade will fetch new versions of packages existing on the machine if APT knows about these new versions by way of apt-get update.

  From http://linux.die.net/man/8/apt-get:

  Used to install the newest versions of all packages currently installed on the system from the sources enumerated in /etc/apt/sources.list(5). Packages currently installed with new versions available are retrieved and upgraded; under no circumstances are currently installed packages removed, nor are packages that are not already installed retrieved and installed. New versions of currently installed packages that cannot be upgraded without changing the install status of another package will be left at their current version. [Emphasis mine] An update must be performed first so that apt-get knows that new versions of packages are available.

• apt-get dist-upgrade will do the same job which is done by apt-get upgrade, plus it will also intelligently handle the dependencies, so it might remove obsolete packages or add new ones. See here: What is "dist-upgrade" and why does it upgrade more than "upgrade"?

  From http://linux.die.net/man/8/apt-get:

  In addition to performing the function of upgrade, this option 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. The /etc/apt/sources.list(5) file contains a list of locations from which to retrieve desired package files. See also apt_preferences(5) for a mechanism for over-riding the general settings for individual packages.

You can combine commands with && as follows:

sudo apt-get update && sudo apt-get install foo bar baz foo-dev foo-dbg

or to get newest versions possible as per version requirements of dependencies:

sudo apt-get update && sudo apt-get dist-upgrade

You need sudo both times, but since sudo by default doesn't prompt you within 5 or so minutes since the last sudo operation, you will be prompted for your password only once (or not at all).

A Google search can give you the definition for almost any terminal command, as can --help in the terminal. For example,

apt-get --help

sudo apt-get update essentially has three parts:

sudo

performs the following command with super-user (root) capabilities. Many actions that require modifying system files or installing applications require extra permissions to go through.

apt-get

is a command-line tool which Ubuntu uses to install, remove, and manage software packages

update

is an option for the apt-get program to use which updates the package lists from a server on the internet. The package lists provide the apt-get utility with important information about the software packages that you can install using apt-get. apt-get uses these lists to determine which software to install when given a command to install. For example

sudo apt-get install guake

would install the Guake terminal as it is currently listed in my computer's local software lists. This may not, however, be the appropriate version, or if the program is new, it might not be available at all. Thus, when installing software with apt-get, you typically type

sudo apt-get update
sudo apt-get install <package>

ensuring that apt-get knows to install the most recent version of the package.

Another useful source for information is the help.ubuntu.com site. For example, if you searched that site for apt-get you would find AptGet/Howto as one of the results.

Running sudo apt-get update simply makes sure your list of packages from all repositories and PPA's is up to date. If you do not run this command, you could be getting older versions of various packages you are installing, or worse, dependency issues. If you have just added a PPA and have not updated, nothing from the PPA will work at all as you do not have a package list from that PPA or repository.

In a nutshell: It is highly recommended to run sudo apt-get update before installing, but it may be skipped if you are really pressed for time unless you have changed repositories or PPAs since the last apt-get update.