The $ usually keeps the sharing as internal to that computer and does not let other computers share.
Sorry, Richard - just have to correct this:
The $ simply makes the share hidden. You can make any share hidden by putting a dollar sign after it. It is perfectly simple to connect to the C$ / D$ or any other admin share over a network - \\servername\c$ and supply the admin (or domain admin) password.
Te NR Fairy is quite correct in saying that the OS automatically creates the C$ etc. shares as admin shares.
SD