South Orange County went through a significant growth period starting in the 1980's. In the early 1990's land developers needed a way to fund the extraordinary amount of public improvements that were necessary to support this growing population. They created a funding mechanism called CFD's - Community Facilities Districts (Mello Roos Districts). Bonds were created to fund these public improvements. These bonds get paid back over the life of the bond - typically 20-30 years - in the form of a special assessment included in the property taxes. These special assessments vary from community to community, but normally range from approximately $25 to $350 per month.
There is conflicting information and opinion on whether Mello Roos taxes are tax deductible. Generally only "ad valorem" (based on value) property taxes are deductible. Mello Roos taxes are not ad valorem, they are flat taxes.