References don't need to be submitted with your application initially, although they'll be an important part of the hiring process and should be prepared in advance. Having two or three references is ideal, with more senior positions often required more.

Preferably, your references should be previous employers. It goes without saying that you should have a good relationship with them. Be sure to contact your references in advance to let them know you'd like to use them, and who will be contacting them (if possible).

If previous employers aren't available, then colleagues (hopefully with some seniority) are another option. There be a clear and demonstrable way in which that contact can vouch for your work and ability.

For recent grads, professors can act as references, although it's not as impactful as someone you've worked with. Most employers understand that your professors have a lot of faith in all their students and want them to succeed, and that work ethic in class doesn't directly translate to work ethic in the field.

References from volunteer organizations are valuable, as long as you've been fairly involved with the organization and for a reasonable amount of time. Spending several hours a month with the organization over a couple years is compelling; having volunteered for one day at an event is not.