There's a large and active online homeschool community. While a local support group will often be your best source for advice about dealing with your neighborhood school board, the online homeschooling community is larger and more diverse than all but the largest metropolitan support groups. If you have a question about how or what to teach, your best bet will often to be to ask it online.
When you send a message to a mailing list, the list sends a copy to all the subscribers. (Some lists let anyone do this; others only forward copies of subscribers' posts.) When people reply, they can either reply to you or to the whole list. Some lists have hundreds or thousands of subscribers, from all over the world; others have a narrower focus.
When you subscribe to a mailing list, you get all the mail sent to it. Most mailing lists offer a digest option, so that you only get a few big messages instead of many little ones.
In general, mailing lists are where you find a sense of online community: Since it's a bit harder to subscribe to a mailing list than to start reading a news group, people tend to feel that they've made a bit of a commitment to a mailing list, that they belong. This makes them a bit less likely to 'flame' and a bit more likely to go out of their way to answer questions. Some of these mailing lists are more specialized than others, but all are devoted to homeschooling.
News groups are a bit different. When you post a article on a newsgroup, your "news server" will send a copy to a few other news servers. These, in turn, send the article on to other servers, until it's made its way to all news servers that carry that group. This can take a few days.
Unfortunately, news groups are sort of like bathroom walls: Post your thoughts for all to see, and come back later to find 27 clueless replies and, perhaps, one insightful comment. This seems to be a consequence of the open nature - anyone can read any newsgroups - and is rather unfortunate, as newsreaders are generally better able to handle a high volume of discussion than email readers. Moderated groups can be better, but much depends on the quality and speed of the moderator.
Discussion boards are much like newsgroups: They usually feature several top-level messages. Each message may have several replies. Each reply can in turn have its own replies, and so on.
Because the whole discussion is stored on a single server somewhere, there is none of the "propogation delay" that can plague newsgroups. Because there is no Big List of all boards the way there is with newsgroups, the boards may suffer from less of the spamming and general obnoxiousness that newsgroups suffer from.
Conversely, the user interface on the typical newsreader is a lot better than the user interface on the typical Web board. The newsreader has better editing facilities for posting new messages, and Web boards typically display the whole "tree" all at once: You can't switch between looking at all topics and looking in detail at a single thread the way you can with a newsreader. Also, because there is no Big List of all boards, the only people in the discussion will be those who were invited or who found a link to it.
The Web-based discussion board's big advantage is history. Because the whole discussion is stored on a single server, everyone can check the context when someone restarts a thread that may have been dormant for months. With email that's stored on individuals' computers, people may or may not have saved the other messages in that thread; news servers typically purge old news to make room for the incoming flood in days or weeks, if not hours.
Chat "channels" or "rooms" are also very popular. With these, everything you type is sent out to everyone else in the room as you type it. This gives discussions an immediacy that mailing lists and news groups don't have.
You can spend a lot of time talking online.