A McJob is pretty much what the name implies. It is a low paying McDonalds type job. Most Walmart jobs are McJobs as well as grocery stores and nearly all fast food restaurants. McJobs are faceless. This means big coprorations will pay X amount of dollars independent of who fills that job. They don't care who or what you know as long as you do the job.
The bigger question is are McJobs a good thing? The obvious answer is Yes & No. However I feel that there is more YESes than NOs.
Why No? This is because McJobs are almost always deadend jobs. There can is absolutely no or very little chance to work your way up the McJob foodchain. If you do go up the McJob foodchain the amount of time taken could easily be spent doing other things.
Why Yes? Because it give McPeople a place to work. McPeople are uneducated, unskilled or transition people such as students or immigrants. Now that may sound harsh but what you have to realize is that this can be the means to a non McJob (such as the case of McStudents).
The best example that I have found is the current riots in France. France's unemployment rate is 10%. This is because France makes employers, ESPECIALLY FOREIGN CORPORATIONS, such as WalMart pay excessive amounts for even the simplest jobs. If you let an employee go, the employer has to pay around 18 mos. of the employees salary or more. So basically these McEmployers don't want to come into country to nearly the same extent.
What does this have to do with the riots? The people rioting where muslim communities that had a 30% unemployement rate. While the riot's intial spark was not the unemployment rate and subsequent lack of money, it was the fuel. A lot of these people had nothing to lose in rioting. They were beat down and had no hopes of climbing out.