yes. Cookie's can be set to be set to delete themselves on a given date/time. They can be set as 'volatile' in which they get deleted when you close your browser. They can be set permenently as well.
All temporary files including cookies are special files. You cannot manipulate them in the normal way. If you want to modify them, I don't know how you can do it... easily (/me has the beginnings of a wicked idea if it's worth modifying a cookie - hrm, maybe for insecure websites). But usually, all you can do is delete them, or move/copy them to another location.