No, the NAS box contains a CPU, embedded system (usually linux), and hard drives. It's like a server in a box - except it's the size of an external HD, not a tower. You connect the NAS to any port on your network... likely connect it directly to an open port on the back of your router. Then configure the NAS through a web page - it's like a configuring the router.
The NAS shares the HDs as volumes (format as one or many partitions) on your network.
NAS is "Network Attached Storage"; it's one or more hard drives in an external box that is directly attached to the network, instead of being attached to a server, inside a server, or directly to a user's computer.
If you just want to connect storage to the ATV, then an external USB (with hack) is cheaper. The NAS will serve the ATV and your mac, and be configurable without going through the ATV.
A NAS box costs more than an external FW/USB drive. If you want/need to share files to multiple users/computers/devices, then NAS is an easy to setup, easy to use solution that runs on it's own.
NAS shares to Mac, Windows, and Linux computers. Performance varies by manufacturer and product - check reviews before buying, as usual.