Is Siri for iPhone an app?
Yes and no. It is an exclusive application for iPhone 4S only,
and it comes with it already, as the other factory apps such as
Camera, Phone, Messages, Facetime, ect.
You can't download it through App Store, and you won't need, if
you have an iPhone 4S, as you will already have it installed.
If you own an older iPhone or iPod Touch version, you can
theorically have Siri installed, but only if your iDevice is
Jailbroken. Search what 'jailbreak' is on google :)
Hope I helped.