Running eth without any argument will synchronise your node to the public blockchain. It is also possible to create or synchronise to another blockchain (see custom blockchain using eth).
Interacting with your node can be done using either geth or the ethereum console:
> geth attach //attach geth to a running eth node.
Using the ethereum console
The ethereum console is a node.js application which connect to a running eth/geth node and provide access to the web3 object.
It can be installed using npm: