According to the web definition, vouchers enable families to use public tax dollars to pay for their children's education at a public or private school of their choice. Voucher programs may or may not include private religious schools. There are positive and negative externalities when it comes down to providing vouchers to families. I personally believe vouchers could be positively effective if they are equally distributed among those that truly need it.
A positive argument for vouchers is that now the "poor" parents can have the choice to send their children to private schools they were then unable to afford. If rich parents have a choice of schools for their kids; poor parents should have the same choice also. There are various religious and non-religious schools available. Unfortunately, the private schools are not free. They are often very expensive. Rich parents often choose the school that has the best reputation. However, poor parents who can't afford the private tuition usually have only one option, the public school in their area. That school may not necessarily the best school academically. Is it fair that only rich parents can send their children to the best schools?.
Similar to the above argument, private schools have a better history of getting results in teaching information and values than public schools. Private schools can be significantly expensive. Yet, even with the cost, parents will usually choose private over public schools. Why? It's because the reputation and results of private schools are so much better. Private schools have a responsibility; if they don't do a fine job, they won't have any students (unlike public schools which will have students no matter how bad of a job they do). Private schools are allowed to be more flexible in their teaching methods. Most of all, private schools focus more on teaching lifelong values that are often tied to religion. With all this said, vouchers will give parents who normally aren't able to afford a private tuition, the chance to send their child to a private school.