An Open source software is the one whose codes are available free and is open to all. Anyone can get those codes, modify them and use for coding similar kinds of application.
In an IT industry the same thing happens. Once a developer gains knowledge in programming, he doesn't sit and right 1000 lines of codes but searches for similar kind of codes and modify them to his requirement.
Open source development has attained its peak as more number of codes on various applications are available online. This has come to break the copyright issues. many online communities are there to guide the young developers so that they get in touch with experts and analysts in that field.
Google has come out with an establishment for open source development known as the Google Summer of Code that offers stipends to those willing to do projects to their mentor companies with the open source codes available.This is a good way to earn money and alo make learning process to be more practical.