I would suggest that you steer clear of rent to own situations.
Most rent to own "purchases" result in the "buyer" walking away with several thousand less than when they started and just a little bit further from home ownership.
There are basically 2 types of rent to own offers.
The first is a lease option. This is where a potential buyer pays an agreed upon sum of money for the option to purchase a property for an agreed upon price, within a certain time frame. i.e. A buyer and seller agree on a purchase price of $100K, the buyer pays the seller $5K for the right to purchase the property for $100K within 2 years of the contract date. If for some reason the buyer isn't able to execute the contract within 2 years then all monies are forfeited and they would have renegotiate the sale price with the owner.
The other type is an owner carried contract. This is where the home owner acts as a bank and allows you to pay them some agreed upon down payment amount and then you pay the owner monthly payments for the duration of the contract.
If you're set on purchasing a "rent to own" home I would strongly suggest you consult an attorney before hand.
Many homeowners sell their homes using one of these methods, when they still have a mortgage on the property. If this is the case most mortgages have a due on sale clause, meaning that if the home is sold in any fashion they demand payment of the outstanding loan balance in full.
If you're set on buying a home in this fashion you would be much better off to purchase a home with the owner carrying the contract. As long as you make your payments the home will be yours eventually.