There are normally two types of contracts. Teachers are paid either by the hour (meaning paid for only the contact hours actually teaching - all marking and planning to be done on your own time). Other contracts pay a monthly salary meaning that when you are not teaching you are expected to be at the school planning lessons, editing, supervising students, or whatever else the school's director asks you to do when not teaching a class. The salary can be a good deal as it is guaranteed and has benefits. One should be careful, however, to come to a firm agreement regarding exactly how many 'contact hours' are expected. Some schools offering full time jobs with salaries will add classes to your schedule at random which can mean no time left for preparation, and often exhaustion. When you are paid by the hour you know at least that more hours teaching = more money.
So look carefully at those ads that promise a monthly salary and "20-25 teaching hours guaranteed". This could mean that they will have you on your feet teaching 4-5 hours per day OR MORE for the same pay . . .