Each survey is classified as either Anonymous or Non-anonymous. You can choose this option while creating the survey.
An anonymous survey is used to get feedback from the users allowing them to give prompt feedback anonymously. No user-identifiable data will be recorded for the responses they will provide including usernames, IP addresses, etc.
A non-anonymous survey is something that allows you as a survey administrator to know who is giving feedback and what feedback he/she is giving. In this survey type username(if the user is logged in), IP address and country details are recorded.
Please note that an INFORMATION DISCLOSURE notice is displayed that will let the respondents know about the survey type before they respond to the survey.
Private surveys are hidden from the front-end surveys listings on the Surveys Listing menu item. The surveys can be accessed only using Unique Survey URLs and Global Survey URLs. Those URLs can be created/accessed under the Invite page of the survey.