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.

#apt-get update

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.

Where as

#apt-get upgrade

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.

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apt-get update will update your local copies of your repositories’ package data, such as available versions and dependencies.

• This is needed to check whether any updates are present.
• It doesn’t actually upgrade packages.

apt-get upgrade and apt-get dist-upgrade will upgrade packages.

• The former runs general system upgrades
• The latter will apply higher level patches such as kernel upgrades.

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• apt-get upgrade will not change what is installed (only versions),
• apt-get dist-upgrade will install or remove packages as necessary to complete the upgrade,
• apt upgrade will automatically install but not remove packages.
• apt full-upgrade performs the same function as apt-get dist-upgrade.

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I typically upgrade my machines with:

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

Below is an excerpt from man apt-get. Using upgrade keeps to the rule: under no circumstances are currently installed packages removed, or packages not already installed retrieved and installed. If that’s important to you, use apt-get upgrade. If you want things to “just work”, you probably want apt-get dist-upgrade to ensure dependencies are resolved.

To expand on why you’d want upgrade instead of dist-upgrade, if you are a systems administrator, you need predictability. You might be using advanced features like apt pinning or pulling from a collection of PPAs (perhaps you have an in-house PPA), with various automations in place to inspect your system and available upgrades instead of always eagerly upgrading all available packages. You would get very frustrated when apt performs unscripted behavior, particularly if this leads to downtime of a production service.

upgrade
    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. Packages currently installed with new
    versions available are retrieved and upgraded; under no
    circumstances are currently installed packages removed, or packages
    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. An update must be performed first so that
    apt-get knows that new versions of packages are available.

dist-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. So, dist-upgrade
    command may remove some packages. The /etc/apt/sources.list file
    contains a list of locations from which to retrieve desired package
    files. See also apt_preferences(5) for a mechanism for overriding
    the general settings for individual packages.

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yum -y update && yum -y upgrade

update:

If run without any packages, update will update every currently installed package. If one or more packages or package globs are specified, Yum will only update the listed packages. While updating packages, yum will ensure that all dependencies are satisfied. […]

If […] the --obsoletes flag is present yum will include package obsoletes in its calculations – this makes it better for distro-version changes, for example: upgrading from somelinux 8.0 to somelinux 9.

upgrade:

Is the same as the update command with the --obsoletes flag set.

Syntax	Description	Example(s)
apt-get install {package}	Install the new package. If package is installed then try to upgrade to latest version	apt-get install zip apt-get install lsof samba mysql-client
apt-get remove {package}	Remove/Delete an installed package except configuration files	apt-get remove zip
apt-get –purge remove {package}	Remove/Delete everything including configuration files	apt-get –purge remove mysql-server
apt-get update apt-get upgrade	Resynchronize the package index files and Upgrade the Debian Linux system including security update (Internet access required)	apt-get update apt-get upgrade
apt-get update apt-get dist-upgrade	Usually use to upgrade to Debian distribution. For example Woody to Sarge 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.	apt-get update apt-get dist-upgrade