Roughly it's like Tor (though I think the transport is not necessarily like Tor), except it's closed. You can only communicate with others on the I2P network. Some I2P users may choose to operate an "outproxy" and provide Internet access but it's not built into the network or software. What is (or was, it's been awhile) built in is a torrent client.

It's also written in Java and the router web UI was pretty snazzy from what I remember.