Let's take the U.S. Internal Revenue Service as an example. I'm going to use somewhat non-technical terms.

This is very simplified but, generally speaking, the auditor examines the taxpayer's records (or other records) to determine the correct amounts of income, deductions, tax, etc. The auditor generally is not involved in trying to obtain the actual tax payment. At the IRS, the top people who work as auditors (or examiners) are called Revenue Agents.

The tax collector comes into the picture much later  after the amount of the tax has been determined. The collector isn't trying to determine the correct amount of tax; the collector is trying to obtain payment of the tax, or to seize assets to sell to satisfy the tax that the IRS claims is owed. At the IRS, the top people who work as tax collectors are called Revenue Officers.

If you compare Revenue Agents and Revenue Officers, a Revenue Agent (auditor) is likely to know more than a Revenue Officer (collector) knows about substantive federal tax law: what items are income, what items are not income, what expenses or other items are deductible, what items are not deductible, what credits are allowed, how the tax is properly computed, etc.

The Revenue Officer is likely to know more than a Revenue Agent knows about procedural federal tax law: how a federal tax lien arises; what things the IRS has to do to perfect the tax lien as against the taxpayer's other creditors, what rights the taxpayer to a hearing prior to seizure of assets, how to legally seize assets, what kinds of assets may be legally seized without a court order, what kinds of seizures require a court order, and so on.