I2P, or the Invisible Internet Project, is an anonymity network. It's kind of like Tor. They both route internet traffic anonymously by bouncing it through multiple computers with multiple layers of encryption. There are a few important differences between the two, however. I'll assume here that the reader is already familiar with Tor, so I'll use that as a frame of reference.

This is not a post about why one is strictly better than the other. I think that they are both valuable tools that suit different purposes.

Key Differences Between I2P and Tor

This is by no means a comprehensive list of differences between Tor and I2P.

Clearnet vs. Darknet

Tor is designed to be used to access the regular internet (clearnet); the exit node makes a clearnet connection. By contrast, I2P is a darknet; by default, all connections begin and end within the I2P network and are encrypted end-to-end.

(Tor, of course, has a darknet as well, and there exist I2P nodes called "outproxies" which can be used to provide limited clearnet access after routing traffic anonymously through I2P. Generally speaking, however, I2P is intended only for darknet traffic, while Tor is intended for both anonymous clearnet and darknet traffic.)

User Participation

Generally, Tor users do not act as nodes in the network. Tor users and Tor nodes are different things. By contrast, I2P users run nodes in the network by default and route other users' traffic as well as their own.

One important consequence of this is that I2P works much better if you can run it all the time. For that reason, I recommend running it on an always-on server.

Ease of Use

Tor is easier to get started using, both due to the availability and ease of use of the Tor Browser and to the fact that users aren't expected to run their own nodes all the time. I2P involves more configuration to set up and use, and it works best if you keep it running. (See the previous point.)

I2P Address Book

You might be familiar with Tor onion addresses, which end in .onion and provide the public key for the site. These are the domains for services on Tor's darknet. Likewise, eepsites (sites on I2P's darknet) have long base32-encoded addresses, which end in .b32.i2p and provide the site's public key. Unlike Tor's onion sites, I2P also provides a mechanism for making these addresses memorable: users can associate the long public key address with a shorter address that ends in just .i2p. For example, the official project homepage has the b32 address udhdrtrcetjm5sxzskjyr5ztpeszydbh4dpl3pl4utgqqw2v4jna.b32.i2p, but it's known as i2p-projekt.i2p.

You can subscribe to other people's address books so that you know about sites they know, and there are "jump" services to lookup which b32 address corresponds to a given short address.

Most eepsites simply reference themselves and other sites by their short addresses.

I dislike this system, as it seems to promote unnecessary centralization (where a small number of jump services effectively control the records of which sites have which names), but it certainly is more convenient if you need to remember an eepsite's address.

Torrenting

Users should not use Tor for anonymity when using BitTorrent. There are lots of ways your true IP address can leak while using BitTorrent over Tor, and it puts a strain on the network.

By contrast, torrenting is one of the supported uses for I2P. The reference I2P software even contains a built-in BitTorrent client called I2PSnark. Note here that I2P provides anonymous torrenting, but only within the I2P darknet, not for regular torrents.

Popularity

Tor is more well-known and has more users; thus your activity within Tor is mixed with more other users. Tor also has more development and research behind it.

Decentralization

While Tor is largely decentralized, it uses a very small number of "directory servers", which maintain a consensus of which relays make up the network. Earlier this month, a denial-of-service attack on these 10 servers took down every single v3 onion service. (Those are the longer onion addresses like nsocnzzkb27qbd2u5dcwglqn6d5epkbxvn4z73eq6xtubzn52n5juyad.onion.) Yes, there are only 10 of these, and half of them need to be up or else all of the v3 onions go down.

I2P does not use such a hardcoded list of central "trusted" directory servers.

Conclusion

Hopefully this post is helpful for Tor users looking for a basic understanding of I2P. Please remember that this is not a comprehensive list. I likely overlooked something you might think is important to know, like that I2P tunnels are unidirectional and short-lived, unlike Tor's long-lived bidirectional circuits or the programming languages used to write the software for each.

For more information about I2P, check out the clearnet site geti2p.net or the eepsite i2p-projekt.i2p (b32).

For the Invisible Internet Project's overview of differences between the two, see geti2p.net/en/comparison/tor or the eepsite i2p-projekt.i2p/en/comparison/tor (b32).