What is the difference between salary payable and salary expense?

Gross earnings are recorded as Salaries Expense. It encompasses the employees net pay and all withholdings (income tax, FICA). If the employee is to be paid at the time the entry is made, you would credit cash for the amount of the net pay. If the employee is to be paid at a later date (probably within the current year or operating cycle), then you would instead credit Salaries Payable. When the employee is finally paid, you would debit salaries payable and then credit cash.