1. Well, the host is what stands in front of the domain name. Usually this is "www". Technically it's a subdomain and this is how you create one in ISPConfig. Consider having a domain "mydomain.com" and want to create a subdomain "test", then you would type "test" in the field host and "mydomain.com" in the domain field.
2. Uhm.. you enter den [domainname].[tld] in the domain field of the site. If you also want the content to be available via another domain name you have two ways: Add a co-domain to your site or create antohter site and add a redirect to it's Apache config. The latter is better in terms of SEO, but of course you can achieve the same effect with a rewrite rule in an .htaccess file for a single site with co-domain(s).
3. AFAIK that field has no technical function. It's only for internal purposes, bcause it could be that a reseller has his own website which is not hosted on your server. So maybe you'd like to add this information at that point to have all customer/reseller data together.
4. You do not have to have resellers, other than in Confixx for example. In ISPConfig an admin user can also create customers and websites. Therefeore reseller accounts are mostly useful if you really want another person to be able to create customers and websites and/ or for security reasons.
The name you enter is AFAIK only for display. I give customer accounts the name of the person or whoever pays the bill. I name the websites after their domains, making it easier to look up the right site. This maybe not so easy if you hav a site with many co-domains. But from an SEO perspective there should always be only one domain per web being displayed in a client's browser (the main domain, if you like) and so I name the web after this one.
5. You're welcome.
At the beginning I found it a bit confusing dealing with the folders. If you don't care your web gets displayed in a top level folder and not where you might want it to reside. You can move it easily, so that's not a real problem. It just shows us all that there is room for improvement and because issues like this have been addressed in the past (see freature requests sub-forum) I'm confident that ISPConfig 3 will have a lot of improvements.
Right now the current stable version is not perfect, but it does not crash or corrupt data and it does well what it is supposed to do for free. The community is alive and well and development seems to make good progress from what I read. It's definately different from other solutions, but it's pretty powerful and reliable.