Dear Lakshmi,
There are no parameters as such. However, you can consider that there are some employees whom the company employs on a regular basis and pays salaries. They are issued appointment letters, etc. You can refer to them as on-roll employees.
Some employees are hired for day-to-day work for a day or a week; these are daily wage or casual workers.
Some employees are hired through a contractor. You pay the contractor a fixed amount, and he disburses their salaries. These are contract workers.
I hope the above explanation is clear.
Regards,
Viraj