Any field in the "Contact Fields" drop-down will be associated to the corresponding field in a contact record. (Examples: Phone Number, Company Name, Position Title)
The Tag Comb and Tag Checkbox fields are great ways for your contacts to auto-segment themselves by selecting their own interests as it relates to your products or services. Whatever they select will be merged into their record as a tag. (Here is a quick article specific to Tagging on Forms)
Any field with the "Custom" title will merge into a Custom Field on the Contact Record. (IMPORTANT: You MUST have already created the fields you would like to merge in your Account Settings > Customizations >Custom Fields.)
Make sure click the gray gear icon and fill in that last box with the Custom Field you would like this information to be entered into (see pictures below).
Social Networks, Advanced Fields, and Survey Fields are NOT added into your Contact Record, and are for your use only. You can view the data entered in these fields from the contact record (clicking into the form from their activity feed) or from a notification you've added as a form automation.