thanks for the response, conspicuous. After following your instructions, I'm realizing that I think what I'm looking for is "Web Forwarding", which as I understand it is the ability to take a domain and have it link to any folder anywhere. In other words, if I have a main site, "www.Tools.com", I can purchase a bunch of related domain names— such as www.Saws.com, www.Hammers.com, www.HandyMan.com, etc.— and not need to build separate websites for them. Via web forwarding, I can have my registrar company (in this case, dotearth.com) "forward" each of those sites to either the main Tools.com, or to any subfolder in Tools.com.
If I understand this correctly, then I certainly don't need anything more than one main domain. I could even place a completely unrelated folder (aka directory) inside of Tools.com—say, Religions.com— and create a whole new website inside that folder. I could then purchase one or two or twenty related domain names (muslim.com, christian.com, etc) and have them all point to the Religions.com folder.
This still leaves me wondering what my 15 subdomains are for!, but I think their purpose is for when you want to create emails that contain ... actually, I'll stop there, because I'm not sure what subdomains are good for. In any case, I'm good for now, and will continue to learn from this forum and the FAQs.
Scott at hhw