The Reusable Content Editor Web Part will allow you to edit a selected peice of reusable content for a site. To add one to your web page, you will need to be in edit mode, and then click the Add Web Part button at the top of the content area of the page.
After you click it you will be presented with the following menu.
Select Cmich from the Categories area and select Reusable Content Editor from the Parts area, then click the Add button.
The Reusable Content Editor Web Part will now be at the top of the page content area. To configure the Web Part, click the down arrow on the Web Part Banner.
Click Edit Web Part, this will bring up the web part settings area on the right side of the screen. The configuration of the web part is in the Miscellaneous section, so click the plus next to it.
This is where you will name your reusable content, please use a meaningful name that is unique and recognizable, as you will need to enter it on any page that will reuse this content.
You can then click the web part content area and add any content that you would like to reuse. This will then be replicated across all of the web parts that contain the same name.
An example of this is the top and right side of this page, as you can see the text is replicated in both areas.