When you return the car, it must be in perfect condition. In addition to having no changes to it, there can be no damages to the interior or exterior. If there are any damages or anything wrong with the car, you will be charged some hefty repair costs. Going over the top will result in a fee. There are also very heavy penalties for early termination of the lease, so be sure that this is what you want to do.
Many people prefer leasing to buying a car.
However, for those who desire to own a car, it can be a more comfortable experience. Once the car is paid off, it's yours to sell, trade, junk, etc. You can make changes to the vehicle, even during the financing period. You can put in a CD player, paint it, tint the windows, whatever you'd like. There are no maintenance or mileage requirements.