Audacity is a free, easy-to-use, multi-track audio editor and recorder for Windows, Mac OS X, GNU/Linux and other operating systems. The interface is translated into many languages.
Audacity is free software, developed by a group of volunteers and distributed under the GNU General Public License (GPL). Free software is not just free of cost (like "free beer"). It is free as in freedom (like "free speech"). Free software gives you the freedom to use a program, study how it works, improve it and share it with others. For more information, visit the Free Software Foundation.
Programs like Audacity are also called open source software, because their source code is available for anyone to study or use. There are thousands of other free and open source programs, including the Firefox web browser, the LibreOffice or Apache OpenOffice office suites and entire Linux-based operating systems such as Ubuntu.