One of the many possible Software Licenses.
Not the same as Free Software. The Open Source Initiative is a marketing program for free software. It's a pitch for "free software" on solid pragmatic grounds rather than ideological tub-thumping. The winning substance has not changed, the losing attitude and symbolism have.
They define what makes a software license open source and provide a list of compatible licenses. Eric Raymond provides an easier-to-read summary as part of his UNIX programming-culture book.
I wonder how much of the rise of OpenSource was (a) the Dot Com bubble making companies more comfortable with sharing/donating work on lower-level infrastructure, and then (b) ex-Dot Com unemployed Hack Er-s with time on their hands (continuing through the Jobless Recovery)? (counter: Geeks With Kids)
Pragmatic issues:
no OpenSource license has ever been tested in the courts
At the moment I'm a little confused about how license restrictions apply when building a product on top of an OpenSource chunk of code. Plat Form vs application? I asked about this for OSAF/Chandler.
requirements for releasing changes to code only seems to apply if you "distribute" the code. What if you use it for a Web App?