A Shared item is a piece of content you create once but can repeat anywhere on a page, anywhere in a site.
Create a shared item
Create a new Shared item in a site's Globals/SharedItems folder.
When you create the item, Sitecore will populate it with placeholder text that is meant to tell you how to go about inserting the item on a page. Don't worry about it.
Just open the item in the regular Sitecore editor and replace the placeholder text with your content.
Display a shared item
Here's what the shared item looks like once the content is in place.
To add this somewhere in the site, basically, you use a dollar sign, the word SHARED (in upper-case), then the location of the shared item in Sitecore. In this example, the item is in /Sitecore/content/about/Globals/SharedItems/ and it's called calendar.
After the shared item has been created, you simply go to each page or add-on where you want to display the calendar, and in the HTML editor find where you want the calendar to appear, put this:
Note that the path starts after /Sitecore/content/sites.
You will have to preview the page to see the shared item. It won't be visible in the Sitecore editor.
Also, remember to check in and publish the shared item, as well as each page that you're displaying it on.