You need to provide more information since your numbers are inconsistent.
Most loan agreements call for payment of a fixed amount (say $1000) at periodic intervals (e.g. 1st of every month), with part of the payment going towards the interest charged and part towards repayment of the loan amount (principal).
Each successive payment has less going to interest and more towards principal
than the previous one
You might have owed $12703.03 as of 12/01/2000 on which the interest was $10.48 = 12703.03*0.0099/12 = $10.48. So on 01/01/2001, your $1000 payment went to pay that interest, and the rest of the payment $989.52 reduced the amount owed to $11,713.50.
The $11,713.50. owed as of 01/01/2001 would have generated an interest due of $11,713.50*0.0099/12 = $9.66, not the $14.64 that you show as paid on 02/01/2001.
So there is additional information that you are not telling us. The interest shown on 03/01/2001 is more consistent with the downward trend in the amount allocated towards interest.
As to your question, interest always accrues and so if on 04/01/2001 you made a principal-only payment of $1000, the interest accrued between 03/01/2001 and 4/01/2001 would add to the loan amount still due. In any case, paying off your loan in huge chunks will reduce the total interest that you will
pay, but your question
What is difference if I pay on my principal? I am not charged any interest right?
indicates some confusion. If you make a principal-only payment, you will not be charged interest any more on the amount of the payment, but interest charged earlier on this amount will remain unpaid, and you will be charged interest on the interest that you have not paid.
Also, you need to look at your loan agreement. Making a payment off-cycle
generally does not relieve you of the responsibility of making your
monthly required payment, but what the off-cycle payment (or extra
amount paid along with on-cycle payment) does do is change the
split of the next
payment, with less going towards interest payment and more towards
principal repayment. So yes, making additional payments is generally
beneficial, with the caveat that if you have other debt
e.g. credit-card balance, also, then you should apply your
extra payment towards the debt with the highest interest rate.