Rome's Army

I found the information of Roman army from
http://en.wikipedia.org/wiki/Ancient_Rome.
The military part.
One of its part talk about that the army had
five parts:the three lines of manipular heavy
infantry (hastati, principes and triarii), a force
of light infantry (velites), and the cavalry
(equites).
We seldom hear about the Roman navy.It was
used to control and defence the piracy.