You generally have two different entities to worry about: your ISP, and the media companies looking to quash illegal downloading. Here's what each of them monitors for and how you can keep yourself anonymous.

Your ISP Sees That You're Using BitTorrent, and Might Throttle Your Connection

In general, ISPs these days aren't so interested in what you're downloading. They leave that to the folks being stolen from. Instead, ISPs are more concerned with how much bandwidth you're sucking up, and whether that's slowing everyone else down. As such, many ISPs will throttle your connection—that is, slow it down—if they see you're using BitTorrent. They don't usually look at what you're downloading (even though they could, if they wanted to), but they will check what kind of traffic is coming from your machine. That is, they'll see how much of it is email, web browsing, video chat, online gaming, and so on. If they see any BitTorrent traffic, they'll slow it down—it doesn't matter whether you're downloading a legal Linux ISO or Batman Begins. All they care about is that you're slowing down their network.

The Media Companies Sees What You're Downloading (and Will Tell Your ISP)

The real problem, if you're downloading illegal media, is the company from whom you're stealing. They (or lawyers or companies on their behalf) actually go online and seek out torrents of their material, whether it be movies, music, TV shows, or anything else, and will download the torrent themselves. From there, they can see a lot of information about the other users connected—including their IP address. You can even check this for yourself at home. Start downloading a torrent and click on the "More Info" section of your torrent client. You'll see the IP address of everyone you're downloading from and uploading to, plain as day.

Once they find your IP address (which they can do just by clicking "more info" in their torrent client), they'll find out who your ISP is and send them a letter. Your ISP then, in turn, will forward you a notice that you've been caught pirating media. Usually the first offense is just a proverbial slap on the wrist, though if you're a repeat offender it could mean having your internet service terminated. If you're very unlucky it could even mean paying a lot of money in a settlement.

So What Should You Do to Stay Anonymous?

It's a dark time for BitTorrent. A lot of the old methods aren't very useful anymore. Applications like PeerBlock claim to block the MPAA and RIAA from connecting to you, but they're not very reliable, and you can still easily get caught when using PeerBlock. Similarly, while your BitTorrent client's encryption can be helpful against throttling, it doesn't always protect you, since some ISPs use more powerful methods of seeing what you're downloading that can get past basic BitTorrent encryption.

These days, the only way to truly keep your downloading anonymous is to take more drastic measures. If you're worried about getting caught downloading illegal materials, use a proxy like BTGuard. It funnels all your BitTorrent traffic through another server, thus keeping your IP address hidden from anyone connecting to your BitTorrent swarm. Even if you're downloading a torrent that's being tracked, they'll see BTGuard's IP, not yours, and BTGuard doesn't keep any logs of their service, meaning they won't trace that IP address back to you.

If you want to keep your traffic from being throttled, you can try enabling encryption in your BitTorrent client. if this doesn't work, BTGuard provides an encryption program along with its proxy service that can hide your traffic better than uTorrent and other clients, to ensure you don't get throttled.