Web Hosting

 Domain

Definition: "The first-level set of domain names are the top-level domains (TLDs), including the generic top-level domains (gTLDs), such as the prominent domains com, net and org, and the country code top-level domains (ccTLDs). Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names that are typically open for reservation by end-users that wish to connect local area networks to the Internet, run web sites, or create other publicly accessible Internet resources." (source: Wikipedia)
Example: http://how-to-blog.tv or http://www.orangecaster.com (note: the subdomain shows up between the http:// and the domain name in the second example.)
Page Rank: Though Google tracks page rank at the actual "web page level" a domain can be considered the equivalent of your social security number on the web as all pages and page rank are generally associated with a domain. (see below for more details)

Subdomain

Definition: "The Domain Name System (DNS) has a tree structure or hierarchy, with each node on the tree being a domain name. A subdomain is a domain that is part of a larger domain, the only domain that isn't also a subdomain is the root domain.[1] For example, "mail.example.com" and "calendar.example.com" are subdomains of the "example.com" domain, which in turn is a subdomain of the "com" top-level domain (TLD)". (source: Wikipedia)
Example: hhttp://blog.macsales.com/ (note that How-To-Blog.TV has no "www" meaning our blog doesn't even have the common subdomain of www associated with it.)
Page Rank: Since each subdomain is treated as a different domain, subdomains will not share page rank with the website. However you can link between the website and subdomain and leverage your page rank this way. It does take time for the "link juice" to pass.
Search Engine Results: Again each subdomain is considered its own website so your search engine results could include one to two results for each subdomain right? Wrong, Google now treats subdomains the same as folders for their search results. (source: SEO Round Table

means no need buy new domain

I Have one question,

they say unlimited domain and sub domain

you will get unlimited domain name and sub domain name that can host on that server space  

k

Domains                         Unlimited

Sub domain                    Unlimited

means what

The actual domain is what your website name will be.

For example= www.yourname.com

The yourname.com is going to be your domain. Now for you to use this domain, you need a public IP address.

For example 168.21.21.24. For humans that hard to remember so what you do is you point your domain name to that IP address. So if you go to www.yourname.com or the IP address it will redirect you to the same location.

Now the location is where you will be hosting your website. That's where your actual website will be sitting.

You can Host your own website. For this you will need a web server. Example IIS or apache and a static IP address. The IP will be given to you by your ISP for extra money.

Because you don't want to spend time managing your server, most people get a hosting plan from different vendors. Example: godaddy or Verio.

They will prepare the server for you. They will give you access to their server for you to upload your website. All you have to worry about is pointing your domain name to the proper IP address and then designing your website.

can u explain what is diff bet domain and hosting

Go through following articles it will help you

http://www.ernet.in/services/webhosting.html

http://www.registerdomainsindia.in/

https://in.business.yahoo.com/domain-hosting-instructions/