It depends on what feature you're using.
In general, there is no ID, only a CASE number for each record.
If you use mailings, then the SERIAL (person ID) is a random string assigned to the respondent when the email address is imported into the address list.
Additionally how is this data saved and do any other people can see the data?
The data is save in a MySQL database, and others (by means of "third persons") shall not be able to access that data at all. I assume, that was not exactly your question? What information, specifically, do you need?
Have you already seen the privacy information on the homepage?