Explanation:
Exponentiation is
a mathematical operation, written as an,
involving two numbers, the base a and the exponent n.
When n is a positive integer exponentiation corresponds to
repeated multiplication; in other words, a product of n factors
of a:

Just as multiplication by a positive
integer corresponds to repeated addition

The exponent
is usually shown as a superscript to the right of the base. The exponentiation an can be read as: a raised to the n-th power, a raised to the power [of] n, or possibly a raised to the exponent [of] n, or more briefly as a to the n. Some exponents have their own
pronunciation: for example, a2 is usually read as a squared and a3 as a cubed.
Negative integer
exponents
By definition, raising a nonzero number to the −1 power
produces its reciprocal:

One also defines

for any nonzero a and any positive integer n. (Our solved example in
mathguru.com uses this concept)
http://en.wikipedia.org/wiki/Exponentiation
The above explanation is copied from
Wikipedia, the free encyclopedia and is remixed as allowed under the Creative
Commons Attribution- ShareAlike 3.0 Unported License.