How does torrent traffic differ?

So I'm not exactly well versed when it comes to networking, and I read some headlines this morning:

> Rights Alliance claims it has has gathered data on 51,000-75,000 users of the site.

- How do they exactly know you're torrenting files over the internet? Does your traffic somehow have a big "Hey, I'm torrent traffic"?

- How is this traffic between a mesh of computers different from your regular browsing?

- How is this traffic different from legitimate torrents (like LibreOffice torrents, p2p game patchers etc.)?