Don't they rely on the "voucher system"
No. Vouchers are taxpayers money mainly used to send kids to private schools, which is ridiculous in my opinion. Private schools aren't held accountable at all for what they teach. Charter schools are public schools that aren't held to the same standards as the districts are...they have to meet the criteria stated in their charter instead.
If charter schools weren't big cash cows for the for-profit companies (and even for the non-profit companies via tax breaks), I might feel differently about them. As a public school teacher, there are a lot of things that I wish were not part of our education system, the biggest being high stakes testing.
The data on charter schools shows that as a whole, they don't really perform much better than public schools. A clarification of an earlier post: they have to take everybody or do a lottery for acceptance. However, they can weed out the lower performing students in a multitude of ways whereas public schools have to accept and KEEP all students.