well, I actually don't even know how to describe what they are other than exactly what they sound like. Like, using us as an example for an art trade, you might look at my art, notice what I can and can't draw, and think of something you think'd look good in my style, and I would do the same to you. With requests, I'd just ask you to draw something, and you'd do whatever style or size (fullbody, headshot, etc) that you choose
Does that make sense? I'm sorry, I'm really horrible at explaining things so you might want to ask someone else or look it up on Google.