Listen to Kirk J. Stark (UCLA) discuss the difference between a tax and a fee.  From the UCLA Press release:

Professor Kirk Stark, an expert in tax law, discusses the difference between a fee and a tax in reference to a newly proposed health plan by California Governor Arnold Schwarzenegger. The California Supreme Court has decided which is which, but the governor and politicians may have very different views as they debate this health plan, which imposes billions in new charges that the governor calls fees, while others call these charges taxes.

Press coverage:

• When are "Fees" Not New Taxes?, L.A. Times
• Gov.’s About-Face on Healthcare "Fees" Is More Than a Matter of Semantics, L.A. Times
• all It a Tax, Now It’s a Fee: Arnold’s Gross Dishonesty, San Diego Union-Tribune