Hoover Dam is what is called an arch-gravity dam. The name curved-gravity dam or arched dam might also be applied. This type of dam uses a curve or arch which has it's apex or "top" directed...
No, but it is the highest arch dam in the US. It was also the biggest dam back in 1935 when it was built. The tallest dam is Rogun in Tajikistan. The largest dam is Three Gorges in China.