Both of these words refer to people who work at a company - but staff is always singular and uncountable - it describes the entire group of workers as one thing. Employees is plural and countable - it describes the collection of individual workers.
The entire staff was happy about the extra day off.
All the employees were happy about the extra day off.
Note that "staff" takes the singular verb was, and "employees" takes the plural verb were.
Whenever you use a specific number, use employees or staff members:
In research, resveratrol demonstrated the ability to improve the dilation of blood vessels, which may allow blood to flow more easily through blood vessels. It has also been shown to relax the walls of the blood vessels, making their diameter larger, thereby lowering blood pressure, and allowing a higher volume of blood to flow through all areas of the body, delivering increased oxygen and nutrients to the bodys cells. Another study found that just 10 mg of supplementary resveratrol resulted in a dramatic reduction in heartattack risk factors.