What is the difference between a Mentor and a Senior Supervisor?

This person would normally have done your job, a job very similar to yours, or most of the duties related to your job before. Your mentor therefore has an understanding of the challenges and responsibilities you will face in completing your job. Your mentor "teaches" you and "fades out" so that eventually, you do the job on your own without their help, guidance, encouragement, etc. A mentor usually does not get paid specifically for being your mentor - they have to complete their job tasks in addition to helping teach / support you in yours. This person usually has an understanding of the company's quality and/or quantity standards that they will address with you at some point in your "training". Your mentor optimally should be unbiased towards you, because they should not have any preconceived notions about you and whether or not you will fit into the company. They should get information from the source - YOU - before starting as your "mentor".

They are being paid to do a job, and part of their job duties includes being responsible to ensure all employees under their direction are completing in a competent way all the job duties and tasks they are assigned to do in a timely fashion. They in turn have to report to someone above them how you are doing in your job, whether or not you are efficient, organized, competent, etc. Whether or not you perform your job duties reflects on their abilities as a supervisor to a degree: do they make every effort to ensure you know what your job duties are and do they make sure you are trained in performing them if you are not meeting company quality and / or quantity standards.

The senior supervisor often has information about you based on your resume, job interview, etc. that could cause them to have preconceived notions about you and whether or not you will "fit in", etc.