Par-boiled (aka converted - as in Uncle Ben's Converted) rice is partially or fully pre-cooked. It's also generally fortified with some minerals (zinc and/or iron). It takes less time to cook. Has less starch, so you get individual grains instead of sticky rice and that's why it's used in restaurants. As an aside, it can have a lower Glycemic Index value compared to white long-grain rice.

-Grainlady

If you bought it at an Asian/ Indian grocery store, par boiled rice is usually short grained rice that has been steamed with the husk on (paddy is steamed). Some of the endosperm nutrients will transfer in. I have found this to be different from ncle ben's type rice(that is if grainlady means the instant 10min type). This is often done in southern india and the flavor is nutty and had a specific fragrance that some people love and others not so much. Par boiled rice is great for making fermented rice/lentil batter used to make Idli and Dosa.

My mom always used Uncle Ben's rice. Although I usually fix brown rice nowadays, I still use Uncle Ben's for Greek stuffed grape leaves. It's the perfect rice because it stands up to lengthy cooking. The grains stay firm and don't get mushy. I cook the stuffed grape leaves in a pressure cooker, and the rice is always perfect. Even the few times I've cooked them in a dutch oven for an hour, the rice doesn't get mushy. I really dislike smooshy rice.

For the same reason, Uncle Ben's is also great for stuffed peppers or other vegetables. I bet that's why it's considered a good rice to use in a slow cooker. However, it's not the best for cold salads because it's so firm to start with. And when reheating it, you'll need to add a bit of water to keep it from being too dry.

Isn't instant rice made up of rice pieces? Sort of like Pringles are made of potato flakes?

No....
Instant rice is great for putting into salt shakers in humid climates to absorb water....but not much else,
Instant rice is rice that has been cooked and dried.