Web designer may be the one who design the appearence of webpages.

Web developer is to develop the complete web site, including the functionalities, cgi, database, etc.

But I find people always get confused with the above two, even some employers who are looking for new employees.

After a web site is developed and put into use, it's the Webmaster's job to maintain the server, keep the message updated, etc.

Please point out any mistakes, thanks.

Sailor's answer is to the point. I have a little bit appenda. Some small companies don't
have a total design at the very beginning, so
the developers create the web site according to their imagination. Before the site goes live,
they may grab a graphic person who will make the page looks beautiful.

If you are cooking up your resume, you
had better put web developer as your objective. In my resume, the objective
is "Seeking a senior-level Java/Web developer position". This is quite vague but
I won't miss opportunites.

As for the WebMaster, there are some specific requirements. Unless you really have WebMaster experiences, don't say you seek a webmaster position. Otherwise, you might miss a lot of developer positions.

Hope this can help. --Jabber