*Open source * is a term that originally referred to open source software (OSS). Open source software is code that is designed to be publicly accessible---anyone can see, modify, and distribute the code as they see fit.

Open source software is developed in a decentralized and collaborative way, relying on peer review and community production. Open source software is often cheaper, more flexible, and has more longevity than its proprietary peers because it is developed by communities rather than a single author or company.

Open source has become a movement and a way of working that reaches beyond software production. The open source movement uses the values and decentralized production model of open source software to find new ways to solve problems in their communities and industries.

So What is Closed Source Software?

Closed source software can be defined as proprietary software distributed under a licensing agreement to authorized users with private modification, copying, and republishing restrictions.

Or in layman terms, the source code is not shared with the public for anyone to look at or change. Closed source is the opposite of open source. 

Closed source is actually the sort of arrangement that you would expect from most businesses, protective of their product and keen to maintain control over their brand and the user experience offered to their customers. Think Apple rather than Android, windows rather than linux.

Platforms used for collaboration:

• Github: It provides hosting for software development and version control using Git. It offers the distributed version control and source code management functionality of Git, plus its own features. 
• Gitlab: GitLab is a web-based DevOps lifecycle tool that provides a Git-repository manager providing wiki, issue-tracking and continuous integration and deployment pipeline features, using an open-source license, developed by GitLab Inc.
• BitBucket: Bitbucket is a web-based version control repository hosting service owned by Atlassian, for source code and development projects that use either Mercurial or Git revision control systems. Bitbucket offers both commercial plans and free accounts.

Popular Open Source Softwares:

1. Linux - Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds.
2. Android - Android is a mobile operating system based on a modified version of the Linux kernel and other open source software, designed primarily for touchscreen mobile devices such as smartphones and tablets.
3. Git - Git is a distributed version-control system for tracking changes in source code during software development. It is designed for coordinating work among programmers, but it can be used to track changes in any set of files. Its goals include speed, data integrity, and support for distributed, non-linear workflows
4. Apache HTTP Server - The Apache HTTP Server, colloquially called Apache, is a free and open-source cross-platform web server software, released under the terms of Apache License 2.0. Apache is developed and maintained by an open community of developers under the auspices of the Apache Software Foundation
5. Firefox - Mozilla Firefox, or simply Firefox, is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. Firefox uses the Gecko layout engine to render web pages, which implements current and anticipated web standards.
6. Python - The mission of the Python Software Foundation is to promote, protect, and advance the Python programming language, and to support and facilitate the growth of a diverse and international community of Python programmers.
7. VLC Media Player - VLC media player is a free and open-source, portable, cross-platform media player software and streaming media server developed by the VideoLAN project. VLC is available for desktop operating systems and mobile platforms, such as Android, iOS, iPadOS, Tizen, Windows 10 Mobile and Windows Phone.
8. LibreOffice - LibreOffice is a free and open-source office suite, a project of The Document Foundation. It was forked in 2010 from OpenOffice.org, which was an open-sourced version of the earlier StarOffice.
9. Atom - Atom is a free and open-source text and source code editor for macOS, Linux, and Microsoft Windows with support for plug-ins written in Node.js, and embedded Git Control, developed by GitHub. Atom is a desktop application built using web technologies.
10. TOR Browser - Tor is free and open-source software for enabling anonymous communication. The name derived from the acronym for the original software project name "The Onion Router". 

Getting Started with Open Source:

Learn Basics of Git and Github from here

Hope you got to learn something new related to open Source : )