Nowhere did I see this spelled out in layman's terms, so here goes.
I own a bunch of domain names, and here's one that is parked, right now: violetweedconsulting.com.
The DOMAIN NAME is violetweedconsulting.com, that is also the 'root domain', it includes the TLD extension.
The ".com" is the TLD extension of that example domain name.
Some other potential TLD extensions are ".edu", ".org", ".net", ".ca" (for canadian domains) and of course TODAY there are a lot more TLD extensions, but primarily, for most uses, those basi ones are all you'll use in the USA.(UK has co.uk, and Australia has co.au, etc.)
Subdomain: The 'www' that comes just before the domainname.com is an example of a 'subdomain', or 'third level (the 'period' . is a separator character).
If you are a newbie, think of the 'www' as a file folder name. If you had a website, you could be (though not always) an 'administrator' for that website on your hoster's website. Then you'd have a login to, for example, www.godaddy.com and you would thus have a login to godaddy's hosting area for your website. On that hosting site you'd have the ability to add different 'folders' like 'search' or 'blog'. The search folder might have a search-engine page (I wouldn't bother with that folder but some sites do, if they have a lot of data, for example). The 'blog' folder would be where your blog posts would go, and you can also redirect a wordpress.com (or .org) or a blogger.com to the subdomain blog.domainname.com, so that you can use the subdomain 'www.domainname.com' for whatever you want and the blog subdomain just for your journaling. There's more info available, but that's the gist of it. www is a filefolder, so is blog., etc. and each subdomain can be 'restricted' to specific users with logins, etc.