Download+ is a way to download plugins, with some extra benefits. Download+ makes it easier to manage your plugins by showing which of your plugins have been updated since your last download, what has changed, and more.
When you're on plugin pages, use the Download+ button to download plugins, instead of the regular download button. Once you've finished downloading plugins, you'll be able to use your HTML-Kit User Assistant account to manage your plugins.
When you click the Download+ button, it'll prompt you to login to your HTML-Kit User Assistant account, if you haven't already. Once you login, the plugin download will continue as usual. Download+ will add the name and the version of the plugin to your User Assistant account. You'll then be able to use that list of plugins to check for future updates and access other Download+ features.
No, Download+ is just an optional feature. You can continue to use the regular download simply by clicking the regular download button on plugin pages.
Update notices will be available in several optional modes: through email notices, as RSS newsfeeds and on the User Assistant page. While the ability to quickly check for updates is one of the important aspects Download+, there will be other features, such as references customized to cover the plugins you use, and more.
No, there's no difference between the plugins download using the regular download method and plugins downloaded using Download+.
No, Download+ uses your online HTML-Kit User Assistant account to manage your plugins. It does not install any software on your computer.
If you have a question that's not answered here, feel free to post it on the support forum