References are optional. If asked, give.

References are basically your boss, and/or your client. You can put your colleague if you've been working with him closely for some times.

If employer calls your reference, they will ask about you, your work, your attitude, etc. This is why you should never put your friend/family as references because they don't know your work.

E: Good morning Mr R, do you know Mr A?
R: Yes
E: Can you tell me about projects that A has been working in the last few months
R: Project? What project? He's my son/uni friend/housemate
E: (hang up the phone)

This is also why you should tell your current boss that you're currently looking for another job. Ask your current boss if he's willing to refer you.

E: Good morning Mr R, do you know Mr A?
R: Yes
E: He's currently applying for a job in our company, can you tell me about his skills and attitudes?
R: What? He's applying for other job? (hang up the phone)