100^2=10000
1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
Assuming you want them added together, the answer is 1,800.
They are basically interchangeable, though "one hundred" is more precise. "A hundred" could mean 97 or 103, and most people wouldn't care. But "one hundred" means 100, nothing more, nothing less.
One hundred hundredths is equal to 1.