Is openstack public cloud like Google drive, aws where we can store data?? Or something else?
Openstack is something you might install to setup an self-service IaaS platform that manages VMs, storage and network. Whereas the top three cloud providers have written their own proprietary IaaS platforms a small cloud provider, or a large company, cannot feasibly code there own cloud-like management software, so they might setup their “private cloud” using openstack (or, say, a big vendors distribution of it) so that many internal projects can use it as self-service IaaS. To quote Wikipedia with some expansion by myself:


  OpenStack is a free and open-source software platform for cloud
  computing, mostly deployed as infrastructure-as-a-service (IaaS),
  whereby virtual servers and other resources are made available to [internal or external]
  customers. The software platform consists of interrelated
  components that control diverse, multi-vendor hardware pools of
  processing, storage, and networking resources throughout a data
  center. Users either manage it through a web-based dashboard, through
  command-line tools, or through RESTful web services.


People might talk about it being “your operating system for private cloud” which is a bad analogy. In general the term “operating system” is often deliberate misused as it has positive connotations and might indicate to none technical people the broad benefits of investing in a given technology. For example people might also talk about container orchestrators such as Kubernetes or Docker Swarm being “the new operating systems of the cloud” which is a similar poor analogy. 

If a backup project said it provides connectors for AWS S3, Azure Blob, Google Cloud Storage, and Openstack Swift then it would be more clear exactly what it does: write large binary backup files into a cloud store. Such a product could then accurately say it supports AWS, Azure, GCP, and Openstack. Just to be clear I am not saying the the technology you are looking at actually uses those APIs I am making a general point about openstack having feature parity with many public cloud technologies.