You can access content in two ways. You can rent content for a limited period of time, during which you can watch whenever you want. Or you can buy content which will put it into your library and you will be able to watch as many times as you want whenever you want.
No, iTunes is not a streaming service. There is no on-going subscription to be paid, or varying levels service. Simply pay for what you want to watch and you can watch it straight away. This is great for the odd show or movie that your other streaming subscriptions do not have.