While Ubuntu does have a wide selection of software in their official apt repositories, as well as what is available in the official (but unsupported) Backports repository, providing packages for newer KDE software versions (as well as some major applications) is often beyond the scope of these official software channels.

This is why Kubuntu use the Launchpad PPA system for their KDE technology upgrades.

Standard Ubuntu Repository Overview

Important Security Updates

This is where all your security fixes come from.

Recommended Updates

Normal package updates come from here. Note that you will not always get every new KDE software version.

Unsupported Updates

This is the official Ubuntu Backports repository. this unsupported channel mostly contains individual packages "backported' from a newer Ubuntu release, but will usually provide a minor KDE software "point" release if it provides significant bug fixes and stability.

Kubuntu Updates

Updates for Kubuntu releases which are due to go to Ubuntu Updates. Mostly KDE point releases.

Example: Kubuntu Wily 15.10 users were able to test out upgrading from Plasma 5.4 1 to 5.4.2 before those packages were moved to Ubuntu Updates.

Use Case:

Releases that are supposed and qualify to go to Ubuntu Updates
Releases that do not qualify for Ubuntu Updates but have low risk of regression
KDE Applications point releases
Amarok point releases
Calligra point releases

To add:

sudo add-apt-repository ppa:kubuntu-ppa/ppa

sudo apt update && sudo apt full-upgrade -y

Kubuntu Backports

Backports of new major versions of Plasma, Frameworks and major KDE apps for Kubuntu which are not yet tested enough to go to Ubuntu Backports.

Example: Wily users were able to try out upgrading Kubuntu Wily's Plasma from 5.4.x to 5.5.0 without having to wait for Xenial's release.

Use Case:

Releases of new major versions
Releases that are tested but still present a considerable risk of regression (for example due to a large change in the dependency stack, such as requirement bump from Qt 4.5 to 4.6)
Plasma major releases and such that imply major stack changes
Amarok major releases and such that imply major stack changes
Calligra major releases and such that imply major stack changes

To add:

sudo add-apt-repository ppa:kubuntu-ppa/backports

sudo apt update && sudo apt full-upgrade -y

Kubuntu Package Staging

DO NOT USE THIS PPA!

The staging PPA is a build ground for new packages and whole package stacks (such as Plasma), once they are staged they get moved to the most appropriate PPA from above. In consequence this means that staging can contain anything from an update to an experimental package and thus might very easily break your system.

Example: Before a new Plasma version gets published to the Kubuntu Backports PPA, it gets built in the clean environment of the staging PPA, once all packages are built and upgrade testing is done, all of staging gets moved to Kubuntu Backports.

Use Case:

Staging only (PPA must be cleaned first to ensure coherent stacks)