As stated in the introduction apt4rpm will create a local APT repository from an ordinary rpm repository. It's doing this by linking the rpm packages in the rpm repository into an APT repository. After the APT repository has been set up, the APT database will created. The system is now ready and can be used. Without updates from your distributor you'll soon have an ancient system. For this reason apt4rpm can also be instructed to mirror your distributor's download/update server, or any other server you want to keep track off. After downloading the latest updates, apt4rpm will update your local APT repository and you're now able to upgrade your system with a single command.
Once you've created an APT repository you can offer it to others. Those (others, the lucky ones) can use APT straight away without going through the struggle to create a local APT reposittory.