Tor and I2P solve a similar problem (anonymous communication), but they do it in pretty different ways.

• Tor is optimized for accessing the regular internet anonymously. It uses onion routing with a small number of long-lived relays, and you exit back to the clearnet through an exit node. Hidden services (now called onion services) exist, but they’re secondary to Tor’s main use case.
• I2P is designed primarily for internal services (called eepsites, torrents, chat, etc.) inside the I2P network itself. It doesn’t rely on exits the way Tor does. It uses garlic routing (a variant of onion routing with bundled messages), and every participant is both a client and a router, making it more peer-to-peer.