Your loan balance is made up to two components, principal and interest. When you are sent out forms it would be impossible to keep the interest up to date as it accrues daily. As a result, companies list the information as principal balance and outstanding interest. Also not combining the two (which in rare cases they are) is beneficial to the borrower as it keeps from having interest accrued on top of previous interest.
This is important in two cases. Many borrowers will call in wanting to know why their balance has not went down. The reason for this is when payments are applied they go to interest first and then once all the interest is satisfied, the remainder goes to principal. If you have $200 in accrued interest but make $150.00 payment it will show your principal as the same as it was previous to the payment since not all the interest was satisfied first.
The second case has to do with payoff amounts. In order to fully payoff your loan you must pay both the principal and the interest. If you pay just the principal, only a portion will go to principal (since interest must be satisfied) and you will be left over with a balance. You can view the total amount owed by adding principal and unpaid interest on your servicers website or by obtaining a payoff balance.