Tom,
Welcome to DNN! I hope you find it to be as useful a tool as I and the community at large do. To answer your questions:
1) Yes, a skin in DNN is synonymous with a template in other CMS systems. The skinning engine is pretty powerful but it can be pretty complex as well. There are plenty of skinning resources (this forum, for example) out there; just do a Google search for skinning dotnetnuke and then let the fire-hydrant of data turn on. Skins will have a CSS file associated with them if you create it (you don't need to link it manually though, it is automated). You can have a "skin.css" file in the same folder as your skin and it'll apply to any and all skin files in that folder... for more granularity you can create a css file with the same name as your skin file (ie., MySkin.ascx could have MySkin.css associated as well). DNN is chock full of CSS files though so be sure you look into and learn the way it inherits; css files involved are "default.css", "skin.css", "[skinname].css", "portal.css" and any number of css files associated with modules (your question #3).
2) CSS was treated in #1
3) "Content Channels" would be considered a module in DotNetNuke. A module is, for all intents and purposes, a miniature program or applet that you can upload into your portal as a "host" or "super" user and then anybody with Page Edit rights can drop a module onto a page into the "content pages" (a skinning concept). By default DNN has several that are preloaded and another set that you can easily install (included in the installation package but not installed by default). Included in those are a Forum, Blog, Wiki, Text/HTML editor, Links list, and more. Aside from those built-in or what we refer to as "CORE" modules, there are literally hundreds if not thousands of modules available both commercially and free (open source and otherwise). A great resource to find them is the DotNetNuke Marketplace or SnowCovered. One caveat is that you really need to research the modules before purchasing them -- download trials, play with demos, etc; otherwise you might get stuck with a piece of junk.
There are several very helpful people here on the forums who will be more than willing to answer your questions, just feel free to ask away.
Good luck!