I'll reiterate, the files are not invisible and in reality the directories aren't either...it's just that the OS X Finder will not display the unix directories unless requested to do so.
The terminal, on the other hand, will list these directories by default and when asked will display all invisible files as well...the ones commonly refered to as "dot' files because they have a 'period' preceeding them.
There are three 'crontab' files in OS X by default, the file we're discussing, the binary and the manual page.