newtonian gravity - How do we know that Earth is not perfect sphere?

I've just read here that:

Equatorial radius = 6378.16 kilometers. Polar radius = 6356.78 kilometers, so the difference in circumference is 71.1 kilometers. It is not a perfect sphere, but kind of pear-shaped.

How correct is that information and what exactly are Equatorial radius and Polar radius with diagram if possible?