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 SC 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.
This is where all your security fixes come from.
Normal package updates come from here. Note that you will not always get every new KDE version.
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 SC "point" release if it provides significant bug fixes and stability.
Updates for Kubuntu releases which are due to go to Ubuntu Updates. Mostly KDE point releases.
Kubuntu Intrepid 8.10 users would be able to test out upgrading from KDE SC 4.1.2 to 4.1.4 before those packages were moved to Ubuntu Updates.
Backports of new major versions of KDE SC and major KDE apps for Kubuntu which are not yet tested enough to go to Ubuntu Backports.
Karmic users would be able to try out upgrading Kubuntu Karmic's KDE SC from 4.3.x to 4.4.0 without having to wait for Lucid's release.
Backports of pre-release versions of KDE SC and major KDE apps, not suitable for production environments but good for those wishing to help test upcoming KDE features and software.
Experimental software or experimental packages, not suitable for production environments nor for those wishing to help doing regular testing. This PPA should only be used by people who know how to recover a system and are well instructed by the developers.
The Kubuntu tool software-properties gets ported to C++, for initial testing it gets published to Kubuntu Experimental to ensure there are no grave issues that render sources.list unusable.
DO NOT USE THIS PPA!
The staging PPA is a build ground for new packages and whole package stacks (such as KDE SC), 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.
Before a new KDE SC 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.