'Link External Data' allows you to periodically sync up the data in your spreadsheet with data available on the web in the form of CSV files, HTML web pages (tables) and RSS feeds. This is quite powerful if you want to make use of live data in your spreadsheet. Learn more about this feature from the blog posts we wrote as it evolved:
Here is a public spreadsheet that uses the 'Link External Data' functionality to import the latest exchange rates onto a Zoho spreadsheet. These rates can then be used in calculations.