Item Details are a set of options for enhancing the content of your Item.
Item URL (Article URL, Event URL)
The Item URL (labeled "Article" in News, "Event" in Calendar, etc) is an alternative URL for an article besides the Sitemason assigned URL. For example, an article in your blog might actually be a link to another website. Like highlighting coverage of your company in the local paper. You would put that newspaper's article link in the Item URL field.
The link for the Item URL will show up in one of two places depending on whether you have content in article Copy or event Description. If you have Copy or Description, any link to that item will show a detail page for that item that contains a link to the Item URL. If you don't have content, any link to that item will go straight to the Item URL, instead of a detail page.
Additionally, if you wanted to attribute the source, Source Name and Source URL fields are available. An example use of these fields might be if you were linking to a newspaper article, you might put "The Tennessean" in the Source Name field and "http://www.tennessean.com" in the Source URL field.
These fields shouldn't need much explanation, but they're optional fields for the author writing the Item. If there's a single primary author, you may consider putting author details in the list tools Settings > Defaults.
The Behavior fields aren't all that oft used, but they're really nice when needed. You and your web designer will likely decide how these settings affect your site visually, so there isn't a uniform behavior for each, but they're more or less a set of yes/no options for you to set per article.
For example, your developer may program your site so when an article is "Marked as Important" it will display on your home page. Or perhaps when "Marked as New" an event gets highlighted a certain way in your calendar.