Rich Office Client Applications.
In the article, Ted goes into depth showing how to begin programming with Smart Tags:
A smart tag is that purple dotted line that appears under a word in Word, or the purple right triangle that appears in a cell in Excel, and the little "i" button that shows up when the mouse hovers over it, allowing you to click and drop some kind of context menu related to that word. For example, in Word when I type in a phone number, like 916-555-1212, the drop down menu contains "Add to Contacts", allowing me to add this phone number to Outlook's address book. See Figure 1, below, for what it looked like in Word as I typed this paragraph: Essentially, a smart tag works by recognizing certain text within the document, and when recognized, makes the icon and drop-down menu available.See also other content on InfoQ as part of our Java + .NET Integration Featured Topic.