A differential pay schedule means that an employee can work for portions of time during a pay period, and be paid different wages/hour for each of those portions. The wages due for each of the types of work are separately calculated (#hours x rate/hr.), and then all the wage amounts are added up to give your gross pay, for the pay period. However, you will receive a cheque for less than that amount, normally, because of deductions that are applied (certain types of payments to government to fund worker-related programs, insurance, taxes, retirement account money, etc.). The amount you actually receive is your net pay.
So, for example, you might work doing one type of easy task for 20 hours, for which you are paid $9/hr., and 10 hours doing a harder type of task, for which you are paid $12/hr (for a total of $180 + $120 = $300 gross wages). The most common categories of work which would be paid differently than your normal rate/hr. include overtime (defined as per a work contract, but ordinarily any time more than the first 40 hours in a calendar week, and perhaps also hoidays), premium shift work (2nd or third shift, that is, 4 pm - midnight and midnight - 8 pm), a more skilled position than your normal work, and high hazard work.
There are many other types of additions and subtractions to your paycheck, which you will get to know when the time arises! You need to know about them when you start earning money at any job, because even if you are working for yourself (not for anyone else), you may still need to be paying a portion of the money you earn for these various withholdings and taxes.