sorry, i'll try this way:
a nas is a great appliance to share files amongst machines over a local network, providing you do not require concurrent file level access. if you want those files to be accessible from the outside (i.e. your nas is at your home and you want to access those files from work) then you would have to make your local network accessible from the outside. there are a number of ways (protocols) to do this, depending on what you need to do with it.
Web (http) is good if you want to host websites on it. To enable it you would have to run a webserver on a computer on the local network (if you use mac, Apache webserver is inbuilt in the OS) and you would have to purchase a public IP from your internet service provider.
FTP is good if you want to share files between locations and with other people, regulating access to those files on a username/password basis. same goes for this though, you would have to run a webserver/ftp server.
streaming....ouch! that's tough. Configuring a streaming server isn't easy, and you normally need to run it on a server with server OS. in a nutshell I don't think it's viable just to be able to listen to your music library elsewhere than at home.
for web and ftp it's much simpler to just buy hosting from a company. in certain cases they offer media streaming capabilities although i would imagine they would charge a lot for it, and then again the website through which you stream the files would be for you to develop, and can be quite difficult.
I put all this down very "high level", without covering further technicalities and there are a lot more ways to access your LAN remotely but don't think they'd apply to what your trying to do.
I hope it helps...