The term domain can refer to:
a machine or virtual host on the Internet; do not confuse the name of a domain - its "domain name" - with the domain itself
a Windows Server domain, a centrally-managed group of computers using the Windows operating-system
a broadcast domain in computer networking
an application domain - the kinds of purposes for which users use a software system
a software engineering domain - a field of study that defines a set of common requirements, terminology, and functionality for any software program constructed to solve a problem in that field. See also domain-specific programming languages.
a CLR application domain, a mechanism for separating executed applications (similar to a process)
In Database Theory, a data domain is a set of all permitted atomic values.
a workstation operating system called Domain/OS.
a group of computers on a network that share a common directory or storage location.
A shared web hosting service or virtual hosting service is a form of web hosting service where more than one instance of the same web service is hosted on a single physical server. This is generally the most economical option for hosting as many people share the overall cost of server maintenance. Shared hosting is the most common form of Web Hosting avalible in todays web hosting market.
A web hosting service is a type of Internet hosting service that allows individuals and organizations to provide their own websites accessible via the World Wide Web. Web hosts are companies that provide space on a server they own for use by their clients as well as providing Internet connectivity, typically in a data center. Webhosts can also provide data center space and connectivity to the Internet for servers they do not own to be located in their data center, called colocation.
Service scope
The scopes of hosting services vary widely. The most basic is webpage and small-scale file hosting, where files can be uploaded via File Transfer Protocol (FTP) or a Web interface. The files are usually delivered to the Web "as is" or with little processing. Many Internet service providers (ISPs) offer this service for free to their subscribers. People can also obtain Web page hosting from other, alternative service providers. Web page hosting is typically free, advertisement-sponsored, or cheap.

Web page hosting is generally sufficient only for personal web pages. A complex site calls for a more comprehensive package that provides database support and application development platforms (e.g. PHP, Java, and ASP.NET). These facilities allow the customers to write or install scripts for applications like forums and content management. For e-commerce, SSL is also required.
domain-name
The term domain name has multiple related meanings:

A name that is entered into a computer (e.g. as part of a Web site or other URL, or an e-mail address). These names are technically hostnames.
The product that Domain name registrars provide to their customers. These names are often called registered domain names.
A name which is not a hostname used for other purposes in the Domain Name System (DNS), for example the special names used by Session Initiation Protocol (VoIP) or DomainKeys.
They are sometimes colloquially (and incorrectly) referred to by marketers as "web addresses" and "domain name" and "host name" are often used interchangably.

Overview

The most common type of domain names are hostnames that provide more memorable names to stand in for numeric IP addresses. They allow for any service to move to a different location in the topology of the Internet (or an intranet), which would then have a different IP address.

By making possible the use of unique alphabetical addresses instead of numeric ones, domain names allow Internet users to easily find and communicate with web sites and other server-based services. The flexibility of the domain name system allows multiple IP addresses to be assigned to a single domain name, or multiple domain names to be assigned to a single IP address. This means that one server may have multiple roles (such as hosting multiple independent Web sites), or one role can be spread among many servers. One IP address can even be assigned to several servers, such as with anycast and hijacked IP space.

 
from : en.wikipedia.org