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The open-source philosophy

Three pillars of intellectual property

Copyright - creative works: writings, songs, movies, and source code
Patents - inventions of products and processes
Trademarks - company and product branding
Patents and trademarks affect software, but "open-source" is primarily concerned with copyright

The main ideas are standardized by international treaty, but may be subject to national variations
If it really matters to you, hire a lawyer

Works receive automatic copyright, without registration or submitting a reference copy
Copying a copywritten work is generally prohibited
Exceptions exist for "fair use", such as criticism, comment, news reporting, teaching, scholarship, and research
To legally copy a work, you need permission from the author or current copyright holder

Early (1950s - '60s) software was bundled as source code with hardware purchases and wasn't seen as having its own value
Fixes and improvements were freely exchanged, especially among universities
By 1970, companies began to recognize the cost of developing software and start restricting free redistribution
By the late '70s, Berkeley University publishes their first free "Software Distribution" to install on AT&T Unix
By 1980, distributing only compiled binaries without source became more common

Through the 80's, tensions grew between AT&T and Berkeley, until the BSD project finally announced that buying AT&T Unix was unnecessary. Then the lawsuits started...
Growing frustrated with the inability to see source code and understand how systems were working leads Richard Stallman to launch the GNU Project in 1983
Within the first few years GNU publishes the GNU Public License (GPL), and a wide collection of utilities such as emacs, and the foundations of gcc and gdb

Open-source often talks about being "free"
- Gratis: without cost
- Libre: without restrictions
While the software is often without cost, the origins were concerned with the growing restrictions

Remember that copyright is automatic and forbids copying without the owner's permission
Open-source licenses are bundled with the code, and give you automatic permission to use it, if you agree to the terms
If you violate the license terms, or the license is found to be invalid, you fall back to the default ban on copying
This legal theory is known as "copyleft", where copyright law is used against itself. This has been tested in court, and generally found to be solid.

"Permissive" - BSD, MIT, Apache. These impose very light restrictions on your use, such as preserving or acknowledging the original license and authorship
"Strong Copyleft" - mainly GPL. These believe in a very strong sense of community, and that if you take the source code, modify it, and ship a new app, you MUST share your source code changes
There are dozens of licenses, each have slight variations on your rights and restrictions. Be careful of what those are and how they combine with other open software
Strongly consider staying with one of the major licenses if you decide to publish your code as open-source