Many people think that the www in www.mydomain.tld (tld = top-level domain) is a part of the domain name. This is not the case, www is the name of the computer www in the domain name mydomain.tld. When you write www.mydomain.tld in your web browser you call for a web service on a computer connected to the Internet. It could equally well be called zzz, w3, johndoe, computer16 or anything at all. The reason for naming computers with a web service to www is that this name suggests that this computer does indeed have a web service. The address of the computer will consequently be the same as the web address if the computer is a web server. A computer can have several names and belong to several domains, and so the same web could have several addresses. You can also have several webs on the same computer.
The http:// is even less a part of the domain name. It stands for Hypertext Transfer Protocol and the reason for writing it is to tell the web browser to call for the web service on the computer to which you are connecting via the Internet. Most web browsers will put http:// in front of the web address automatically if you do not type it in.
See also under the heading DNS.