Tags are used to group your attendees. By grouping your attendees using tags, you will have the option to print name badges by tag (allowing you to have different designs of badges by tag), you can filter by tag in the Attendee list screen and finally you can register attendees using tags (tag your event and everyone in that group will be automatically registered to that event.
User Defined Fields are used to add more meta data to your attendees. It is a way to capture data in string, drop down or check box. This is helpful for registration purposes where you can ask your attendees to provide more info about themselves. UDF are included in some reports.
I hope this helps and apologies for the late response. Please email us at firstname.lastname@example.org for more info.