Many answers here are correct, open source is free as in you are free to utilise it as you want, as long as you want, and in any way you want including selling it to other people or giving it away for free. I just wanted to add a couple of comments as to some of these perceptions, "free as in free beer", "works on every operating system", "can/can't charge for it" come about.
1) Most open source software is licenced under the GPL, LPGL and MIT/BSD licenses and can therefore be passed on to anyone once you have the source code for no charge. Because of this, you actually can't economically charge for open source software, because there may be a way to get the same software for no charge, just by getting it from someone else for free. However many smaller software developers make a good living off producing open source software because:
- they continually develop and improve the software, possible for a platform - for example a CMS - that is very popular. What you are selling is the support and long term stability of your product in a market that may not have a lot of stability
- they actually do provide good and cost efficient support by email or forum. This can be to end users, but also to wholesalers, such as some libraries do for software developers.
- the niche they occupy is actually quite small, with 3-10 players worldwide
- Some providers simply ask for a reasonable price per installation, even though they have no means to enforce it and many people pay out of thankfulness or moral obligation. This works by the way also for non-open source licence models
So in summary, the software is often also free as in free beer, but again this may not be the primary reason why it is downloaded, bought or used.
2) Open source software often works on more operating systems that closed source, because if the need is big enough, you may have a way to compile the source code yourself and plug any gaps on your platform. For closed source software this decision fully depends on the manufacturer of the software, so it is less likely to happen.
3) Because of how 1) works, many people seem to think that you are not allowed to charge anyone something for open source software. I'm not sure if this is a wrong reversal of causality that is quite common, or some sort of self-deprecating morals that cut you out from valueing your own work. I've been selling custom build software and business solutions by using open source licences for many years and it is a win-win(-win) for all parties involved, even though my customers pay fully for the software:
- My customers can do absolutely anything they want with the software, only excluding stopping me and my upstream open source suppliers to use the underlying modules for other clients. In return they get to use all of the upstream intellectual property without having to pay for it.
- I can provide them security from buying from a small supplier by having control over all deliverables to the extend of replacing the supplier
- They can stop me from aggresive distribution to their competitors by non-competition clauses. In my view, custom software reflects each specific business anyway, so their competitor would be stupid to want it.
- They are free to distribute the software, but of course that would undermine their own investment. So they will only distribute to their successor/ business buyer
Hope this helps,