A shared web hosting service refers to a web hosting service where many websites reside on one web server connected to the Internet. Each site "sits" on its own partition, or section/place on the server, to keep it separate from other sites. This is generally the most economical option for hosting, as many people share the overall cost of server maintenance.
Shared web hosting can be accomplished in two ways: name-based and IP-based, although some control panels allow a mix of name-based and IP-based on the one server
In IP-based virtual hosting, also called dedicated IP hosting, each virtual host has a different IP address. The web server is configured with multiple physical network interfaces, or virtual network interfaces on the same physical interface. The web server software uses the IP address the client connects to in order to determine which web site to show the user. The issue of IPv4 address exhaustion means that IP addresses are an increasingly scarce resource, so the primary justification for a site to use a dedicated IP is to be able to use its own SSL certificate rather than a shared certificate.
In name-based virtual hosting, also called shared IP hosting, the virtual hosts serve multiple host names on a single machine with a single IP address. This is possible because when a web browser requests a resource from a web server using HTTP/1.1 it includes the requested host name as part of the request. The server uses this information to determine which web site to show the user.
DNS stands for "Domain Name System." The domain name system acts like a large telephone directory and in that it's the master database, which associates a domain name such as www.wikipedia.org with the appropriate IP number. Consider the IP number something similar to a phone number: When someone calls www.wikipedia.org, the ISP looks at the DNS server, and asks "how do I contact www.wikipedia.org?" The DNS server responds, for example, "it can be found at: 18.104.22.168.". As the Internet understands it, this can be considered the phone number for the server, which houses the web site. When the domain name is registered/purchased on a particular registrar's "name server", the DNS settings are kept on their server, and in most cases point the domain to the Name Server of your hosting provider. This Name Server is where the IP number (currently associated with your domain name) resides