I have heard of these terms since I worked in the field.

Basically the web designer designs the layout, type and logo design for a website often through html and or Dreamweaver type programs.

The web developer is the person who is responsible for the programming of the website. Yes, the designer is designing in html but the developer is in charge of the more in depth programing for the company. A Web developer can also be a Web designer, but a Web developer typically has more database, CGI, and engineering experience. In short, they are more technical than a web designer.

The web administrator is the person who oversees the website, documents problems and issues with the site, etc. They also sign up users for accounts, oversee security, and manage site performance much like a manager. There is much more to this job but this gives you an idea.

One person can be all of these but many companies hire separate due to size and nature of the job. Big company, more likely separate. Small company, most likely all one job.

Hope that helps a little!