Custom variable

After reading all docs, I still get confused about effectively using custom variables.

I am using _paq and found how easily you can set a custom variable, for example when a user gives his name in a form. But I did not find in the docs a clear and complete example about the retrieving of that variable (getCustomVariable).

Say that the same user comes back to the site, how can I read the cookie (custom variable) with his name? Or the fact that the session is changed makes impossible to retrieve that cookie?

thank you and sorry for my basic question.