Okay, so basically what seeds are. They are coding that is wrote when the game starts or the C-Gear is initiated.
For instance, let's say the seed is a story book. As soon as you open it (starting of the game) it begins to tell a pre-determined story. That's what a seed does, it holds all the information that will happen during that gameplay. So a seed will be what determines your IV's, your PID (nature, ability, gender, etc.), when you encounter a Pokemon. All that jazz.
How a seed is determined is by a certain mathematical formula. Which is rather complex and we don't really need to get into details about it. Just that to hit that seed, you need to start the game at a certain date, and time.