In 4 to 5 sentences!!
It is important to note that Open source basically means the source code used to build the application is available. That does not necessarily mean the source code is free. Corporations that charge for software will charge for the source code too. Organizations (normally non profit) or individuals that provide free software normally will not charge for the software. This is mainly because it is the public community that provides the support (defect fix, documentation, enhancement, features etc) and therefore it makes no sense to charge the “hand that feeds”.
I have used many free open-source software like java, mysql and have not had any issues. Furthermore many major corporations use java and mysql for their important applications. In my opinion it seems the more users use open source applications, the more support is available from individual users.