I need to know what properties do we get for a form authenticated user in sharepoint 2013(which can uniquely identify them like ID), i need to keep track of activities performed by each user who visits the site.
Depends on your forms authentication provider. In general, you would get the properties of the SPUser object, if populated by your provider.
The ID property is assigned at the Site Collection level, so is not consistent across all of SharePoint. In general, use the LoginName property.