A User Repository is a site where any user can submit automated instructions for building packages from source (PKGBUILDS), which are periodically reviewed by Trusted Users. Once a PKGBUILD becomes popular enough, it is distributed as a binary in Arch's Binary Repositories. If you've ever used Mac OS X/BSD's ports system, it is very similar.
There are two major user repositories: The Arch User Repository (AUR) and the Chakra Community Repository (CCR). Cherimoya is only compatible with the CCR, although it only takes minimal modifications to import AUR PKGBUILDs to the CCR.
The Arch User Repository is a PKGBUILD repository for Arch Linux. No binaries can be provided with the PKGBUILDs, although patches are allowed and should be reviewed. The AUR has about 30,000 packages and counting, a testament to the ease of making PKGBUILDs. Few distributions of this size have a comparable amount of packages.
While Cherimoya should be able to use AUR PKGBUILDs, their build process has to be checked over and their dependencies retrieved, as Chakra (our base distro) may have different methods or naming conventions. There is more information on porting here. If you do manage to make an AUR PKGBUILD work, please move them over to the CCR so others can benefit.
When Chakra forked from Arch Linux, the CCR was immediately created to provide the AUR's functions. These packages are usually cleaned of GTK dependencies when possible, and have a few edits to account for the small differences in package naming. Otherwise, they are exactly the same as AUR PKGBUILDs.