What's a Domain Name?
Basically it your address on the web. It is what you want people to remember when they go looking for you on the internet. Like Google http://www.google.com , or Macarldie http://www.macarldie.com only it is a certain length and it has www. in front of it and can have different endings such as .com, .biz, .ca
Blogs have different types of domain names, so that your actually being hosted on their site. Like this site, Angelz Web Design is hosted by Blogger.com http://www.blogger.com and so for me to have my blog on here it http://angelzwebdesign.blogspot.com and it had no www in front. The same goes for Wordpress but with Wordpress you have the option of downloading a template and having it manipulated and then re-loading by FTP- file transfer protocal up on to the internet and that way you can have your own domain name. Wordpress.com http://www.wordpress.com is the same as Blogger.com http://www.blogger.com whereas Wordpress.org is where you actually download a template and upload it to a Webhost and get your domain name.
I hope this simplify things so that you have a better understanding of these terms and below I have given Wikipedia's explanations which are more technical.
http://en.wikipedia.org/wiki/Domain_name Here is a good explanation from Wikipedia
domain name is an identification label that defines a realm of administrative autonomy, authority, or control in the Internet. Domain names are also hostnames that identify Internet Protocol (IP) resources such as web sites. Domain names are formed by the rules and procedures of the Domain Name System (DNS).
Domain names are used in various networking contexts and application-specific naming and addressing purposes. They are organized in subordinate levels (subdomains) of the DNS root domain, which is nameless. 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. The registration of these domain names is usually administered by domain name registrars who sell their services to the public.
Individual Internet host computers use domain names as host identifiers, or hostnames. Hostnames are the leaf labels in the domain name system usually without further subordinate domain name space. Hostnames appear as a component in Uniform Resource Locators (URLs) for Internet resources such as web sites (e.g., en.wikipedia.org).
Domain names are also used as simple identification labels to indicate ownership or control of a resource. Such examples are the realm identifiers used in the Session Initiation Protocol (SIP), the DomainKeys used to verify DNS domains in e-mail systems, and in many other Uniform Resource Identifiers (URIs).
An important purpose of domain names is to provide easily recognizable and memorizable names to numerically addressed Internet resources. This abstraction allows any resource (e.g., website) to be moved to a different physical location in the address topology of the network, globally or locally in an intranet. Such a move usually requires changing the IP address of a resource and the corresponding translation of this IP address to and from its domain name.
Domain names are often referred to simply as domains and domain name registrants are frequently referred to as domain owners, although domain name registration with a registrar does not confer any legal ownership of the domain name, only an exclusive right of use.
The Internet Corporation for Assigned Names and Numbers (ICANN) manages the top-level development and architecture of the Internet domain name space. It authorizes domain name registrars, through which domain names may be registered and reassigned. The use of domain names in commerce may subject strings in them to trademark law. In 2010, the number of active domains reached 196 million.