Screen-sharing tools are a way to let other people view and sometimes control your computer remotely. It's easy to imagine a number of cases where such utilities would prove useful, but they're particularly popular for remote collaboration and presentations, and can also be a great way to provide technical support if you manage a website or software product. There are a ton of screen-sharing options out there with a variety of features, but few are as straight-forward to set up as Join.me.
Most people probably either use screen-sharing software all the time for their work, or almost never. If you're an "all-the-time" user, the chances are good that you already have a full-featured solution that you like (or hate). If you're in the "almost-never" crowd, join.me looks like a great service to keep in mind for those rare times when screen-sharing would prove valuable (and maybe encourage you to find a few more of those times).
To get started with Join.me, just head to their website and click the big button next to "Share." Assuming you're on a Mac or Windows machine (Linux users are out of luck), you'll be prompted to download a file. This file is a standalone application (no installation needed) that will launch a sharing session for your screen. After starting the program, you'll have a new menu on your screen with a link to share your screen. Copy that link and send it to whoever you want to have access, and they'll be able to see your screen directly from any modern web browser with Flash installed. That's literally all there is to it; you're now sharing your computer screen. When you're done with the session, just close the menu, and delete the file (or keep it around for use in the future).
Inside a session, join.me provides a few basic options. You can text chat through an integrated client, send files from your computer to other users, and let other users control your computer directly. There's no integrated voice chat, but it does provide an option for setting up a telephone conference call. The options for viewers of your screen are fairly similar, but also allow one to change the viewing settings to zoom, or see things in full screen.
I ran across a few bugs while testing things out (the chat menu doesn't work in full screen, and neither does mouse sharing), but overall, everything went very smoothly. If you plan on doing a lot of screen sharing, it's almost certainly worth investing in a full featured package, but for the occasional time when you really wish someone could see your screen, join.me fits the bill.