In broad terms, a legal job is one that will be within the realm of the applicable laws and statutes in particular jurisdiction. Generally speaking, any job is considered a legal job as long as no violations of the law are required as part of such a job. All criminal activity that is done for a profit can therefore be considered not be legal jobs.
However, because there are variations in the law itself, there may be certain legal jobs in one state that are not legal in another. An example could be seen in the United States with prostitution. Prostitution is largely considered not to be legal job or practice in the United States, yet its allowed by law in certain counties in the state of Nevada. This particular instance illustrates how a job or practice that is considered illegal in most jurisdictions can be considered to be legal job in others.