The name means different things to different people and the role varies greatly between organisations of various types and sizes. Ask two people and you'll get three opinions, it really is something of a grey area.
If you have a web server installed on your computer at home, and you installed your O/S and web server yourself, and you perform backups, design and build your own web sites, then technically, you are a webmaster. Although you are master of a fairly small domain.
In such a web team, the Webmaster is able to better focus on the Web service and servers and surrounding technologies and leave the other roles to the respective specialists in their areas, but the webmaster's responsibilities will often overlap with those of the other team members.
In smaller organisations a dedicated webmaster would be a luxury. This is where the role and your talents are generally spread the furtherst and where a webmaster must truely be a jack-of-all-trades possibly absorbing many of the other roles normally filled by a web team in a larger organisation.
Often, if required to be involved on smaller web projects, a web master may be project manager, spec writer, programmer, administrator as well as being a kind of mediator or liasion between the client and other project staff such as graphic artists an DBA's.