What is a frequency domain and s domain?

[KittuTarun]

What is a frequency domain and how can we understand the same in practical world?
How is it related to complex plain sight domain?

 

[Svein]

In short: When you are designing amplifiers and wireless equipment, you are thinking and designing in the frequency domain. When you are designing digital circuits, you are thinking and designing in the time domain. These two domains complement each other, what is hard in one domain may be easy in the other.