A domain or a domain name on the Internet is an unique name within a toplevel extension. Exemples are :
Note that there is no http:// nor any www in front of it.
http:// refers to a used transfer protocol, often related to HTML or XML (but not necessarily) while www refers to a node name or a machine/server within the domain. Though www is classical the name to indicate the main webserver within the domain, here also it is not necessarily. When one takes for instance www.alterlinks.com then the whole is no longer a domainname, but an FQDN (Fully Qualified Domain Name). The domainname however, remains just alterlinks.com.
It is not possible to "buy" a domainname but rather "rent" it. To obtain a domainname, one should first check to see if the desired name is available, for instance with a whois tool. If the name is available, it can be registred. Each country has a NIC who is responsible for the toplevel domains in that country. For instance, in France for .fr and .re, it is the AFNIC. Though a NIC is responsible for "their" domainnames, it is most of the times not possible to obtain a domainname directly from a NIC. Instead, domainnames may be obtained through accredited Registrars. Most NICs will have a list available of their accredited Registrars to choose from.
How to choose an accredited Registrar?
The choice of which Registrar to use may depend. A Registrar may impose for instance the fees they're asking for registering a domainname. Therefor, the prices may vary. From experience, it may not always be a good idea to pick a Registrar solely based upon their fees; it may also depend on what exactly you want to do with your domainname. There may be Registrars that may propose registration and a combined hosting for your domain. Other Registrars may let you choose to combine the registration with a hosting possibility or allow you to modify the DNS entries on the domain record. If you have your own DNS servers, or if you desire to host your domainname with a hosting company who doesn't provide Registration services themselves, it may be (very) handy to choose an accredited Registrar who allows you to modify the DNS entries which will enable you to transfer it elsewhere or, better said, change the DNS servers who will be responsible for your domainname. Again other Registrars may allow you to just only point the "www" and "pop3/smtp" entries elsewhere but not the DNS entries. Also the time that a Registrar has been around may be of influence on your choice. As a domainname may represent your identity on the Internet or your company's presence on the Internet, you do not want to take any risks. Once you have a domainname, you usually want to keep it. Don't trust blindly on reminder emails from your Registrar, write down the renewal date, send yourself a reminder alert or anything you can think of to remind you to renew in time.
A (very) lousy Registrar for instance is DotEarth (also known as Get-A-name, who are known for "snatching" domainnames the moment you "forgot" to renew (in which case it would have been nice on their behalf to actually send reminder emails and not to snatch it for themselves)). How one can see that a Registrar "snatches" domainnames for themselves? Very easy, because the domain creation date remains the original, so in fact it was a domain transfer just before it actually expired into the public domain... Got to love Way Back Machine (at least sometimes) where it is visible that DotEarth was one and the same as Get-A-Name.
When confronted with it, why a "snatched" domainname was now registered or transfered rather to Get-a-name as owner, (GETANAME, 1350 E. FLAMINGO ROAD 736, LAS VEGAS, NV 89119, US, firstname.lastname@example.org, Registrar: DOMAIN REGISTRATION SERVICES INC. DBA DOTEARTH.COM, Whois Server: whois.dotearth.com, Referral URL: http://www.dotearth.com) they hurried to say that they didn't know eachother and hurried to put the "snatched" domainname behind a name shielding, making it impossible to see any longer to whom the domain is registrerd except for law enforcement. Alrighty then ... Curious why they did that. Because when looking at the websites of getaname.com and dotearth.com using the Way Back Machine the contact information and even the telephone number are the same. Also the "webdesign" of both sites looks the same in 2003, if not exactly the same. Very curious co´ncidences for not knowing eachother... Some people are lousy in lying and even worse in correct webdesign (or they would have known how to keep their traces out of the hands of Way Back Machine). Some people just love "old" domainnames because they tend to rate better in search engines and can be used to put publicity on, enjoying a lot of external links linking to it, bringing nothing-suspecting visitors. When your own Registrar does things like that, it plain su*ks, avoid them! Run from them!!
An other example to proof a point, a WIPO official Complaint which was lost against DotEarth. When reading this, make sure to be seated well; The final decision was not in favor of the Complainant because they didn't see how the (now) current owner could have registered this domain with DotEarth in 2000 (Creation Date) thus before 2003 and therefor couldn't have registered it in bad faith in 2000 already. But that is the trick, they never did! They don't let domains expire and re-create it (in which case the domain creation date would be a new to date one) but they transfer it just before it expires, hence keeping the original domain creation date.
Some personal revenge? Perhaps, but a warned person counts for two. Unfortunately not all Registrars play fair game, that they're ICANN certified or refered doesn't always mean something.. Anyway, if ever staff of mentionned Registrar reads this, one can only hope that they're smiling as they were on the telephone while confronted with this. Service with a smile so to speak, which you feel very well in a place where the sun never shines.
Not only Registrars of whom one may expect that they play fair game may keep themselves busy with such doubtful activities, also robots automatically search for near expiring domainnames, especially "older" domainnames and try to snatch it the second it expires. Either for the purpose of putting publicity on them, or for trying to sell it back to the now previous owner for big money. It is thus very, very important to renew a domainname in time as chances are very slim to get it back once the domainname changed owner.
A good and recommendable (yes, they do exist) Registrar on the other hand for instance is Gandi SAS. Fast, friendly and exceptionally knowledgeable Support staff and offering a broad range of services, including full control over your own domainname(s).
If a Registrar does allow you to modify the DNS entries and if you want to transfer your whole domain elsewhere (= change the DNS servers responsible for "carrying" the zones), make sure that the new DNS servers are setup correctly to receive it. If such modification is taken into account but the new DNS servers are not set up correctly, your domainname may temporarily "vanish" and can no longer be reached/found over the Internet. Each NIC may have slightly different technical requirements of the DNS zone configuration and the DNS server configuration. Before proceeding in any DNS change on your domain name record, it is very wise to run the future responsible DNS servers through a zone check and only proceed once the new DNS servers passed it without any errors.