Joomla!® is Open Source.
So, what is Open Source and what does this mean.
Generally there are two types of software:
- Proprietary (or Closed Source), and
- Open Source.
Proprietary, or Closed Source software has source code that only the software creator (person/people, organisation(s), or team) who created that software has access to that source code.
Only the original creator(s) (person/people) can legally copy, inspect, or alter that source code.
Open Source software, like Joomla, however is different.
Joomla source code, because it is Open Source allows anyone to copy, inspect, and alter that code as well as redistribute the modified code.
There are often still legal requirements when modifying and re-distributing Open Source software so always do research first before doing so.
Such legal requirements are determined by a Licence.
Such a licence may be referred to as: GNU GPL
GPL stands for General Public Licence and explains the conditions for modifying and re-distributing Open Source software.
Specifically for Joomla, you can read about Joomla Licensing here.
Altering the Source Code (also perhaps known as Core Files) for Joomla and using the altered code is typically done at your own risk.
Altered core code is generally not supported by the original code developer(s) - so if your alteration does not work, or causes your site to no longer function you may well find yourself "on your own" when it comes to sorting it out.
Joomla extension developers will also often not provide support if their extension(s) do not function correctly when core Joomla files have been changed or modified.
You might also be interested in our article Joomla Core to find out a little more about the Joomla core files.