Absolutely. As long as the defining factor that separates groups of employees isn't race, color, age-over-40, religion, or other protected characteristics, employers are legally justified to treat one group of employees differently from another.
The following terms are commonly used to designate various types of employees:
These terms are defined by employers rather than by law. Their definitions can vary from workplace to workplace, and they're primarily used to indicate which types of benefits certain employees are entitled to. You're justified in offering full benefits to one group, and limited benefits to another, for instance.
If you're an employer of both full-time and part-time workers in California, you could choose to offer a sick leave benefit to your part-time workers in compliance with the recent Healthy Workplace, Healthy Families Act of 2014 while offering a more generous PTO benefit to workers who are full-time. If you're a TriNet customer, you can use the free PTO product to track full-time and part-time employee time off.
You're free to offer different benefits packages to different groups of employees, as long as the defining characteristics of the groups aren't EEOC protected traits.