Specifically, I have a recipe that calls for toasted peanuts. What is the difference between roasting a peanut and toasting a peanut? I can find roasted peanuts at the store, but not toasted ones, so I will probably try and toast my own at home, but don't know whether to start with raw peanuts or already roasted ones.
The previous answer which says to toast something means to brown it is accurate. The difference between roast and toast is simple, really: roast means to expose something to dry heat (in the west, usually in an oven) and to cook whatever it is right through; toast means to brown the outside of something, either held over a fire (as in marshmallows) or placed under a dry heat source such as a grill or inside a toaster.
In practice, for your peanuts, its a lot easier to roast them than it is to grill them to get a toasted effect, but toasted should mean you put them under a grill and shake them about to brown them all over rather than 'cook' them right through. The risk with toasting under the grill is, obviously, burning - I've 'toasted' cashews in a hot oven by shaking them about a bit periodically for a short time, and I've also done it in a dry pan on the hob. The only time I did it under the grill, I burnt the lot...
To toast something is to cause it to gain color through the application of heat. That's it. To toast a peanut is just like toasting a piece of bread. It can be done in the oven or on the stovetop, with or without oil. The difference between the words "roasting" and "toasting" is subtle and the words are often used interchangeably, but the true meanings aren't actually identical. You "roast" something that is raw to make it safe and pleasant to eat. You "toast" something to add color and flavor to something that is already safe to eat.
Simple and clear cut, right? Well, maybe not. As a child I always "roasted" marshmallows, I never described the process as "toasting" them. Go figure.
The meanings of the two words are so similar that they overlap in a way that it makes any distinction between them meaningless unless you can crawl into the brain of the author. But you're asking about peanuts. That makes it easier. The fact is that there is no such thing as a raw, toasted peanut. If the peanut is toasty brown, it is also roasted, even if it started the process raw. Get it? So, start with raw peanuts, start with roasted peanuts - it doesn't matter. Once the peanuts are toasted, they are also roasted, so just take it from there and don't look back. Thinking about it too hard can only cause headaches.
In general, I think of 'toasting' as finishing the cooking process in something that has already been through several steps. Toasting baked bread for example. Roasting to me implies taking something from start to finish.
With peanuts, it could be that they expect you to take already roasted nuts and then toast them (perhaps with sugar or something). It also could be that they mean just roasting the raw peanuts.
Toasting is not for browning. Browning can be done I believe in different ways. Intensity of browning gives us an indication of degree of toasting or how far the toasting is done. Therefore purpose of toasting is not for browning.
Purpose of toasting is to heat, warm or dry something, eg. bread, and also to achieve crispyness. Degree of browning will shows how far the toasting is done. If it is burnt it will be black as any other burnt items, not to be eaten as the burnt part will become toxic. Intake of any toxic material will bring cancer. Degree of browning tells us how far the item is toasted. It's up to the person to leave how brown it should be to match his / her taste Also toasting is not purely cooking as well. Roasting is another way of cooking something but with the purpose of achieving crispyness as well. Depends on how workable when it is placed in the own. if the mixed item is with too much of liquid, no crispyness can beachieved at the bottom. When we roast, inner parts also become evenly cooked in addition to the crispyness on top or the portion of the surface not dipped into the liquid part of the mixture. Such process can not be achieved by toasting.
Good thing in both the methods is, we could avoid using oil or any other form of fats as we dosometimes in cooking on the cooker. Some roasting too require few drops of oil or fat to surve the purpose of cooking by the person.