what is Open source software?

Hai Sevin.
It seems too general question actually, Mr. Google also can help you find out a lot of information about this, by the way here is the general definition for OSS:_(adapted from:http://en.wikipedia.org/wiki/Open-source_software)..

"Open-source software (OSS) is computer software with its source code made available and licensed with a license in which the copyright holder provides the rights to study, change and distribute the software at no cost to anyone and for any purpose. Open-source software is very often developed in a public, collaborative manner. Open-source software is the most prominent example of open-source development and often compared to (technically defined) user-generated content or (legally defined) open-content movements".

and below is about licensed that may come with OSS:
-Apache License, 2.0 (Apache-2.0)
-BSD 3-Clause "New" or "Revised" license (BSD-3-Clause)
-BSD 2-Clause "Simplified" or "FreeBSD" license (BSD-2-Clause)
-GNU General Public License (GPL)
-GNU Library or "Lesser" General Public License (LGPL)
-MIT license (MIT)
-Mozilla Public License 2.0 (MPL-2.0)
-Common Development and Distribution License (CDDL-1.0)
-Eclipse Public License (EPL-1.0)

and many more.....

*Tips: When search for information in Mr. Google, it's good practice to use an appropriate keywords.

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