What is the difference between pay and/or reward system in any company?

"Pay"
is the agreed upon or contractual salary or wage for a given position, based on qualifications
and experience in the position.  It is more or less guaranteed each pay period, and as long as
the person does well enough to keep the job, does not go down.

"Reward" or what is more commonly referred to as "incentive"
happens more commonly when people receive a commission for sales or profit-sharing for the
employees of a company. It can also include the year-end bonuses awarded when a department,
individual or company does well.