Tooltips functionality is new in Dynamics CRM 2013, though Field Description property was in Dynamics CRM 2011, it was not meant for tooltip, however in Dynamics CRM 2013 the description text set for fields is now displayed as tooltips when you place your cursor over the labels for fields. Set the description text in Entity field customization to provide guidance for people in your organization.
Following screenshots shows how tooltip is set on Fax field on Account:
Note: Unlike the Description for fields, the Description value for a global option set is not displayed as a tooltip when the field is used in a form. This description is only visible in the list of global options. You can use the description to provide information about why you have created this global option set and what it should be used for.