The rundown:

Famicom Twin Mfg. SHARP 

Comes in Black and Red.

Red was replaced by Black because it was

considered more "manly"....

Hardwired controls.

Incorporates the famicom and disk drive chipsets. 

Feature AV out and RF, SUPERB sound.

Cart slot clock in place during operation to prevent use of "backup" devices

Has a basic flaw, like all famicom disk drive systems. The belt grows loose, and the head no longer moves making the disk side useless.

FAMICOM DISK

FAMICOM CART

Not to Scale (Duh)

Not to Scale (Duh)

The Disks are slightly smaller than 3.5 floppies, but are much thinner.

Famicom disk games in the USA (Nintendo) were released on cart. Zelda wasn't on cart in Japan until 1994.

If treated properly (Stored vetrically, at a constant temperature) disks will last years, until they inevitably loose their magnetism.

Some common USA cart games like metroid are worth hundreds in Japan on disk.

Avoid storage in areas where temperatures fluctuate,and near areas of electrical activity.

Discs are prone to dirt unless stored in special slips. Avoid buying or trading loose disks unless the person offering them is reliable.

The bootlegs: Or, how the Famicom Disk System was killed off...

In an effort to offer lower costs per game, the FDS was implemented. Games were supposed to be available almost anywhere, in stores and convenience stores. What was more interesting, one could buy blank disks and take them to stores and get games installed on them. There were kiosks on the streets where you could purchase games.The games would be recorded, and the drop out. It was possible to put a game on each side if the game did not use the other side for save information.

Almost immediately, pirate disks sprung up. Later, a device was made to fit between the famciom and the famicom disk drive add on (not twin, hence the twin's slot protect device) that allowed the piracy of carts to blank disks. This pretty much finished off the FDS, and games once again started to be made on cart.

This is one of the proven cases of piracy killing off a system before its time.

Now, ironically, with disks demagnetizing, pirate versions of disk games (on cart) will be all that will eventually remains. 

A good way to see if your disks are real, they have the nintendo name and are usually yellow. Pirates are funny, like nintindo, or intedo.