Today the term «open source» is most of the time used similar to the term free software. But actually open source software (oss) means an application with an open source code, what means that all functions of a program are accessible for everybody. So all interested people are able to copy and spread legaly and for free the specific software. Furthermore everybody can view the source code and adapt it for personal customisation. OSS is published mostly under General Public License GPL.

The great advantage by using open source applications in your company is the fact that you don't have to pay regularly any license fees, as you have to, if you're using paid software. Support by using oss you will find in a wide spreaded community of developers and other users: In the large Joomla- and TYPO3-Forums you will find a lot of professionals and fans who can help you at any time and often for free.

Also companys like for example «Start to web», which is specialised in the use of this open source cms, will offer their support. A little disadvantage of open source is for sure that the developing of the specific software is not driven with the same speed like by professional software companies. That's why you can't always expect a bug fixing immediatly. But if there is a huge developing community as by Joomla and TYPO3 , you can be quite sure that technical failures will be fixed fast.

Further information about open source on wikipedia