OK, let me mention some important caveats, just so you can keep them in mind:

You can think of usenet like the internet. You have data on servers all around the world, you have sites such as Google which index these sites and content, and you have your ISP which gives you access to the internet.

Likewise, on usenet you have the data scrambled on servers all around the world, on different backbones of the usenet. These backbones are accessed through service providers for the backbones (sometimes they're resellers, sometimes it's the backbone selling access). These service providers operate just like an ISP, selling you monthly or yearly access to the usenet backbone of your choosing.

Then there's the Googles of usenet, Indexers. There's a ton, they vary a bit from one another, but essentially they find all there is to find on usenet, presenting the files to you as a whole. You want a specific... Ahem... Linux iso? An indexer will know where all the pieces are and it will tell you with an NZB file, kind of like how torrent files tell you where to look. Indexers can be a monthly subscription, but some of them offer lifetime subscriptions as well, and they don't break the bank.

The last bit you'll need is your download client, to do what you do for torrents. These are free tools, sabnzbd and nzbget. Either one works.

So, I did mention there's multiple backbones of usenet. Indexers don't lock themselves to specific backbones, and no indexer covers everything there is on usenet, which means that to get the most out of usenet, you'd ideally have multiple indexers and multiple providers (making sure you don't get providers from the same backbone as they'd essentially have the same data). Multiple indexers give higher chances of finding something on a search, while multiple backbones increase your chances of finding all the pieces needed to complete a file. This is not absolutely necessary, but dare I say you'll notice the difference as soon as you bump things up to 2 of each.

So, essentially, usenet is by far the best method for completing your media library (leaving torrents as a desperate backup route), but it can become expensive.