a.*** x 10^n
where a is an integer between 1 and 9 inclusive, .*** means some decimal expansion, the number of decimals may vary based on how many significant figures you want, and "x 10^n", is read "times 10 to the n" and n is some integer that tells you how many times to move the decimal point to the right or left to get the actual number. If n is negative, we move the decimal point to the left, if it is positive, we move it to the right. So here goes.
8 billion = 8,000,000,000 = 8.0 x 10^9
this means we take the number 8.0 and move the decimal point 9 places to the right to get 8 000 000 000, which is 8 billion
0.00003125 = 3.125 x 10^-5
this means we take the number 3.125 and move the decimal point 5 places to the left to get the number 0.00003125
Understand? if anything is unclear, please say so