Question: I was reading about CMS and ERP. And got confused at this point that whether a system can both be an ERP and a CMS? is it possible?

Answer: 
In a very theoretical sense yes. But in most cases this would not make very much sense. The basic functionality of an ERP system is business management. The data produced here may become part of the data displayed on a web site. But normally the data here is pure text and numbers, like definitions of products, orders, invoices and some statistical information.

An CMS main task is to provide information in a way that not only the content as such can be edited but often enough the representation and styling too. Its functionality is much more limited and the amount of data normally far less than in an ERP system.

In our company for example all our product and customer data is kept in an ERP system. Customers can login and search products, order them and see shipment and invoice data. All this they get directly from the ERP system. Some information in the ERP is specifically there for the web site (for example our product categories for the web site are slightly different from those used internally for statistical purposes). But this data is purely information. In theory this ERP system would have a lot of options to store even more information, even combined with some styling information. We don't use this.

Information that we do not actually need for business purposes is edited in a small CMS system for the web site. This includes things like a news section, some pages with company information and similar more text/style oriented data. We could store this in the ERP too, there are tables for such things, but it is more effort to code in the ERP area and then we would have more effort to display on the web.
An example, though maybe a contrived one but one I encounterd in the wild a long time ago:  

An application was built that would store incoming documents pertaining to required reports sent in by customers of a financial services company.  

The same application also served to automatically send out reminder letters to customers who were late sending in those documents, and as an entry point for the telephone support people in the company to look up customer information if customer called (or had to be called).  

In that case both functionalities were closely intertwined. In other systems both functionalities might exist but not be as tightly linked (think something like SAP or Lotus Notes).