i think for a Persian learner, these terms can be confusing. Referring to your question, they simply have the same meaning in spoken and writing. In our city, Isfahan, we use these terms interchangeably. It is noted that برنج is only used when one wants to buy it from supermarket and پلو is used when he or she is in restaurant. Isn't it easy ?

In summary, "برنج"is used for both,raw rice and cooked rice.But "پلو" is only used for cooked rice.

most of the time we use برنج when we do not cook it but when we cook it we use پلو for example when we mixed rice and lentil and cook them we say we cook عدس پلو when we mixed it with green beans we say we cook لوبیا پلو or when we پلو with a kind of خورشت we say we prepare پلو و خورش but when you want buy fore example a kilo of rice you have to say "من یک کیلو برنج می خواهم"

I have to mention that برنج is more general and you can use it for every kind of cooked rices but it is more correct to call some of them which ballboy mentioned with پلو and چلو another important point is that for raw rice you have to say برنج. besides there is two important way to cook rice in Iran the formal way is a little more complicated and they let the rice to get taller and thicker in boiling water then bring it out and then shed cold water on it. it is called the process of shocking the rice and have some points in its taste and shape this process is چلوکش کردن (chelockesh kardan). then put it again in an oily pot then put it on very small fire until vapor comes out of the pot lead. idiomatically we say دم بالا داده which means it produce vapor. this kind of cooked rice is called پلو یا چلو and even some times they name it برنج چلوکش another way is as above but without process of چلوکش کردن. they let rice in boiled water until all of its water change to vapor. it is general to call it برنج not پلو or چلو