How to take backup for a particular domain and project in an archive ??
Assuming your repository is not stored in your database (this was disallowed as of QC v10), you have to back up the database for the project and the repository files. Use the backup tool of your choice, appropriate to your database software and server operating system.
An alternative is to export the project to a qcp file, but that works only for small projects I hear.
You can also create a new project by copying an existing project. We keep the backup project deactivated, but it is quickly available if we have to recover something. Your backup method will be determined by how quickly you want to be able to access the backup and how large your projects are. The copy method, like the export method Judy mentioned, takes time for large projects. And of course you need storage space to keep the backups.