Hmm... gnu.org is down for me and I can't actually check these articles to make sure I'm not imagining things, but I will link them anyway:

http://www.gnu.org/philosophy/free-software-for-freedom.en.h...

http://www.gnu.org/philosophy/open-source-misses-the-point.e...

Probably you can understand simply by looking at the urls anyway ;-). To say that F/OSS doesn't refer to 2 different features, "free" and "open source", is to advance only one of the viewpoints from the 2 movements.

As much as it would be convenient if it were not the case, "free software" and "open source" software do, in fact, refer to different (although similar) things in many people's minds.

I am not the right person to explain the differences and if gnu.org were not down I would simply point you to the arguments that the FSF have against referring to free software as "open source". However, I will do my best to explain. Please verify what I'm about to say when you get the opportunity.

The intent of free software is to ensure that all users and programmers can deal with the software on a level playing field. By this I mean that one programmer can not use legal or technical mechanisms to have more capability than another. Everybody has the same access and ability to run the software for whatever purpose they want. They can inspect the source code in an un-obfuscated way. They can modify it in any way. They can distribute it, along with their modifications, to others. Most importantly, they can not do anything that will take away these abilities (either through legal or technical means) from others.

The licenses in the free software movement are a means to that end. The free software movement defines licenses that are free (in other words licenses which do not impose restrictions that would remove the level playing field). Some licenses, though, are free in and of themselves, but do not actually stop people from working in a non-freedom-oriented way (I'm sure I am not the only person frustrated that there is no antonym for "free software" -- all commonly used ones being deprecated by the FSF).

For example, there are many licenses which are free licenses but which allow people to leverage the code in software that is not free. This creates an un-level playing field because those people can build on top of the software, extending it in a way that only they can use and then compete against the original project without being free. In other words, if project A is free software licensed with a permissive license, someone can make A+ which is essentially A but with more features. They can release this under a non-free license, competing against A and taking mind share without giving people the freedom to modify or distribute A+.

For this reason, people in the free software movement prefer some free licenses over others. Their entire reason for having the licenses in the first place is to avoid the situation described above.

The open source movement is not as concerned about the issue described above. While they advocate nearly identical methods, their goal is to encourage licenses that allow people to interoperate smoothly if they choose to do so.

In the open source movement, it is unreasonable to prefer licences that enforce software freedom over other OSI software licenses. In fact, many people in the open source movement consider these kinds of licenses to be undesirable because it stops them from using F/OSS to build products that are not free software -- in other words, they are diametrically opposed to software freedom and only cooperate in a similar fashion when it is advantageous for them.

In my opinion, the term "Free and Open Source Software" is useful for discussing the very large amount of area where the free software movement and the open source movement overlap. It is important not to conflate the two movements, though, because whether or not you value software freedom there are many people who do.

In my own usage, I prefer to use the term free software when I think that the freedom aspect of it is important. I use F/OSS when I am just talking about interoperating with people using free software/OSI licenses. I use OSS when I am talking about leveraging F/OSS into software that is not free.