Open source is a set of principles and practices that promote access to the design and production of goods and knowledge. The term is most commonly applied to the source code of software that is available to the general public with relaxed or non-existent intellectual property restrictions. This allows users to create software content through incremental individual effort or through collaboration. Read More at Wikipedia
Always double check that the code you are using is open source. Some of the code in the links below may not be open source.