How and when do updates of apps get into repos?

[DB]

Hi all,

A question that has scratched at my grey cell for a time.....

There are apps in repos & there are more recent versions on their home 
pages or in Source Forge - how and when do the updated versions get 
incorporated into the repos? 

And the other side of the same question.... If I install/build/compile 
or whatever one of these updated versions into my F10, will yum & co be 
able to keep tabs on it or must I keep a running watch on the page I got 
it from?  

And.... if I find an RPM on a non-Fedora site, what are likely/possible 
side-effect consequences of installing it??

Thanks

Dave