Open-source software (OSS) is computer software with its source code made available with a license in which the copyright holder provides the rights to study, change, and distribute the software to anyone and for any purpose. Open-source software may be developed in a collaborative public manner. According to scientists who studied it, open-source software is a prominent example of open collaboration. The term is often written without a hyphen as "open source software". 

Open-source software development, or collaborative development from multiple independent sources, generates an increasingly more diverse scope of design perspective than any one company is capable of developing and sustaining long term. A 2008 report by the Standish Group states that adoption of open-source software models has resulted in savings of about $60 billion (£48 billion) per year to consumers. 

In the early days of computing, programmers and developers shared software in order to learn from each other and evolve the field of computing. Eventually the open source notion moved to the way side of commercialization of software in the years 1970-1980. In 1997, Eric Raymond published "The Cathedral and the Bazaar", a reflective analysis of the hacker community and free software principles. The paper received significant attention in early 1998, and was one factor in motivating Netscape Communications Corporation to release their popular Netscape Communicator Internet suite as free software. This source code subsequently became the basis behind SeaMonkey, Mozilla Firefox, Thunderbird and KompoZer.
What's one of the reasons for their sharing?
 the early days of computing, programmers and developers shared software in order to learn from each other and evolve the field of computing.