While creating a field in CRM 2011 or CRM 2013, we can provide a description for the field being added. Until CRM 2011 they had more of developer utility and were used to hold content that was focused to developers. However with CRM 2013 these can now be displayed as Tooltip when the user hovers over the field label on the form.
In CRM 2011 when we would hover over a field label, tooltip would just display the label of field. If you would want to display additional information about the field as a tooltip we either had to change the field label or write some unsupported jscript that would interact with the html dom to display the tooltip.
With CRM 2013, Microsoft has overcome this limitation of CRM 2011. Now we can add additional help text about the field to better identify how it should be used which would be displayed as tooltip on the form.
There is an important catch for this, if you are upgrading from CRM 2011 to CRM 2013 all the description for the fields will get updated/upgraded. Also if there was anything that we had left on a field description as a developer notes that we wouldn’t want the end user to see. That would also be visible to the end user. Hence it might be worth a while to go through all the existing fields and provide brief information about what each field is for.
Hope this will help!