Developer- Builds the backbone (behind the scenes stuff for the site), sets up the functionality of the site, etc
Designer- Does logos, color schemes, site design, etc
The Developer and Designer can be the same person, can be part of a package (you hire a company that has a separate designer and developer), or you can hire them separately.
Adding articles/content is easy once you learn a little HTML, and your developer can help you out with easy tasks like that, but when you want to start adding new features, pages, etc, that's when it can get a little more complicated.
You're going to want someone that charges an hourly fee most likely that will tackle projects as you need them implement into the site. The administrator can be someone who oversees the site as a whole or sometimes even just a message board admin. I have a second administrator, who is the guy who helps me with the marketing of this site when I need it, or upgrading the software.
Maybe a phone call would best explain all this
