I2P routes traffic through unidirectional encrypted tunnels, and every participant relays for others. It is built inward-first: hidden services (eepsites) and peer-to-peer applications inside the network, rather than exit traffic to the clearnet.
Compared to Tor it has a smaller network and anonymity set, fewer exits by design, and different traffic-analysis properties. It appears in infrastructure setups where internal reachability matters more than clearnet browsing.