A job is any legal activity that allows an individual to perform a service and in return earn credits she/he can use to buy things.
A job can mean "some work that has to be done", for example: there are jobs to be done in the house: washing up, mending things that are broken, etc.
A job can also mean: work that a person does to earn money. The word "job" may be used when a person works for someone else "an employer" who pays them for the work. For example, a teacher's job is to teach children or adults. A taxi driver's job is to drive people in a taxi. A firefighter rescues people from burning buildings and puts out fires. A dermatologist's job is to diagnose and treat skin diseases. Some jobs pay very little.