The Den Den Mushi are actually snails which use radiowave to communicate which each other. Here is a picture of a wild Den Den Mushi without any devices attached to it:
Here is what its wikia page has to say about it:
They have the ability to communicate with each other telepathically through radio waves. The people of the One Piece world take advantage of this ability by attaching buttons and receivers to them. Den Den Mushi thus take the place of telephones and other similar machines in the world of One Piece. They are classified as type "B" creatures, being "Small Friendly" [...]
The gold Den Den Mushi is a rare kind of Den Den Mushi.Here's its wikia's description:
It is used to summon the Buster Call attack. The Den Den Mushi is immobile and its entire body appears to be made out of gold. It is a very rare type of Den Den Mushi. After the button on its back is pressed, it sends a message straight to the Silver Den Den Mushi, which is the signal for the Marines to start the Buster Call.
Now, all Den Den Mushi are mechanically improved. However, it is not confirmed whether or not the golden Den Den Mushi is clothe in gold even in its wild form (which I doubt).
The golden snail only communicates with the silver Den Den Mushi, shown above. So far, it has been shown to be limited to alerting the Marine about a Bluster Call request from the Silver Den Den Mushi.