Different kinds of PCB design software’s are available in market and according the requirement different users used different software. Like I am comfortable in designing my PCB on Eagle and my vendors who fabricate the PCB does not have Eagle than what will happen? Hence to bridge the gap between the PCB designer and manufacture we use gerber file. Gerber file contain information about the PCB like what will be the pad size, drill size, name of component etc. With the help of gerber file information obtained and manufacture feed the data into their machinery to manufacture PCB. The most common format that is used today is RS-274-X. But before using always check with the board manufacture. It is a good practise to check your layout before manufacturing because once it has been fabricated you cannot correct your mistake. Or it may happen that PCB will not be of any use this will lead to wastage of time and money both.