I'm sure you'll get much more detailed answers than this but just for quick reference:
three letter identifier= VOR,
J and numbers= jet route above 18,000',
five letter identifier= intersection or fix,
V and numbers= Victor air route used by aircraft operating below 18,000',
three letters and numbers or five letters and numbers= arrival and departure routes ie., POLAR2 arrival to DTW, or the EAU8 arrival to MSP.
This is NOT a conclusive list and there are variables, but you get the jist of it.