A few basics most of the explanations miss, plus a couple 'opinions':
Usenet is essentially an 'electronic bucket brigade'. Messages posted on one server get 'echoed' to others, and so forth. The linchpin in all of it is A) how good of an interconnection that server has to the others, and B) how good their retention is. Any message you d/l from usenet, for whichever server, has a 'header' that is fairly 'human' readable, listing where it originated from, the nodes it went through to get to the server you're pulling it off from, and finally (of course) that last server you did pull it off from.
Explanation: ALL material on usenet are MESSAGES. The fact that 99.99+% of the traffic is now files, doesn't change this fact. Your newsreader will tell you how many 'lines' the message has, and the total size of that message. Most/all files have multiple messages (made up of multiple lines). NZB's were 'invented' by Newzbin several years ago to make things easier. If you have qualms about blithly starting up an nzb download, you should use the nzb the 'original poster' uploaded or generate your own (if your newsreader supports it).
Several newsreaders have the capability to hunt out and generate nzb's on their own, or have for pay dedicated servers that can do it for you. There are tons of sites that do this for free as well, as you've probably figured out, plus quite a few that do it as a business ('for pay') like Newzbin itself.
Now, think back what it was like several years ago without nzb's, and even before that when newsreaders couldn't 'tag and download' all the little parts of things. Major headache. But doable fairly easily if the s/w was up to the task.
For those coming from the P2P world, the speed, flexibility, and extremely high completion percentage will be striking. This is what commercialism will do for you. The legal underpinnings of Usenet were well established in the 1980's, and there has been no challenge to any commercial operation that hasn't been laughed out of court.
Some indexing operations (Newzbin is an example), outside the US, that have been scared into jumping through some hoops, along with a least one commercial server operation. But any country (the US) that allows extreme commercial speech (up to and including the buying of votes by commercial firms of legislators) in deference to the 'free speech' tenets Constitution (as the Supreme Court ruled long ago that money=speech), isn't about to lower the boom (in actual court) of anyone's speech over usenet. All the saber rattling you've probably heard about is exactly that: saber rattling. But it's like trying to stomp out fire ants: 'Good Luck'.
That this has all been going on for well over 20 years now, is testament to that. I've been there for all of it.
