On several Ubuntu sites such as OMG Ubuntu there are summaries of new ppa's. These are useful programs, but they don't appear in the Software Center.
However, in the center there are programs that specifically say are "community supported" and not Canonical supported.
So what is the difference between Software Center programs and PPA's?
It would be a lot more useful if more PPAs were in the center.
I believe you are missing some information about PPA's.
After you install a PPA this PPA -is- included into Ubuntu Software Center. You can see all the available packages for that PPA. They are included in software sources as shown here:
but there is also a list of PPA's inside USC (I believe it is under installed).
Besides that: PPA are generally software that are not supported by Ubuntu itself. If someone that created software wants this to be included into USC as a part of Ubuntu they should and can apply for it to be included. Though most do not care for it since this method seems to work best for everyone.
For more information see: What are PPAs and how do I use them?
firstly PPA is personal package archives.
Its like an online account for storing packages, etc, that any individual can create and share with other people.
An owner of a PPA can upload personally created or modified software.
Software Center programs are more like the Google Play store or the Apple App Store application.
Software there is known to be stable release (not beta) versions.
Broadly speaking : PPA packages operate on the basis of trust -- you have to ask yourself how trustworthy is the person who owns it.
And software center applications are recognized or approved by Canonical, so they are the most reliable.
