Sorry, I don't have a background in finance so I might be misusing terms. I was using the two interchangeably. Is credit rating for individuals? My impression is that when the city takes on debt, it issues bonds on the bond market to raise money. The bonds are rated by the three standards agencies: Moody's, Standard & Poors, and the third one. The rating is based on the city's ability to raise funds, as well as different financial ratios (debt to equity, etc.) and is either AAA, AA, A, BBB etc. Based on what the rating is (with AAA being the best) the city would pay an interest rate of prime + x%. So if the city was downgraded from AA to A, any new debt would have an extra (say) 0.5% interest tacked onto it in order to sell. Larger governments have a larger tax base so they usually have better ratings and lower interest rates, since they're basically guaranteed to pay back the principal.
This is my mental picture of how things work, so if anyone knows in more detail I'd appreciate any corrections.