1. In short a motherboard connects all of your other components together (RAM, CPU, hard drives, disc drives, video card, sound card)
2. No it has to be a certain kind or processor that is compatible with that motherboard.
3. No it will only take components that are compatible with it, but you do have many different options for which components you buy since there are a lot that are compatible with any motherboard.
Do you have a certain motherboard you have questions about?