Actually, open source hardware sounds like bollox to me. But its a term Dave Jones of EEV blog used. I assume Open source comes from open source code used in programming.
The real point (in both hardware and software) is to understand the license under which the design (whether it is a schematic or source code) is released.
I'm mostly familiar with GPL. In software, if you release your program as GPL, you are required to provide a copy of the source code with the program, or upon request - and you can't charge more than possibly a small copying fee or media fee (if you were shipping the code via mail and/or in paper or other media format - but today, none of that makes much sense). Anybody using that source code (say they take the code, and derive their own version of the software with changes or additions to the code to make it better) is required by the license they agreed to (the GPL) to make those changes and their source code (with the changes) available, under the same rules. So on and so forth.
The point of this license is to not only encourage sharing of code, but to make it so that any changes (ie, forking of the code) doesn't cause those changes to become "closed" to the outside world. Its a reciprocal value system; whereby if you value the software enough to improve it, then - since you were given the original form of the software, your improvements should also be free for others to use, incorporate, learn from, and expand upon.
Now - that is a very simple explanation of the GPL (and likely there are some errors in it - read the license at gpl.org if you want the real scoop) - and not all open-source licenses are like the GPL; some are more restrictive, some are much more open than the GPL (for instance - the BSD license, basically, only requires that you acknowledge the original authorship of the code, and display the copyright somewhere - other than that, you are free to use it as you like - which is why both Microsoft and Apple are famous for using BSD licensed code, yet hardly giving back any updates or additions they make to the codebase - because the license doesn't require it).
So, you can see that it could (in theory) be possible to release a schematic with GPL or BSD style licensing/restrictions - but so-called "open-source" hardware don't use such a license; I'm not really certain what license is most common (I think the Creative Commons license is fairly well represented, though).
Now - this is all completely different from potential "public domain" or other "freeware"-like published schematics you see on the web today. Some of these may be truely public domain (ie - do with them as you wish with no guarantees of fitness, operation, or safety) - but some may just be "published", but still copyrighted by the author. If you know who the author is, it might be wise to seek clarification on this.
So, in short - check for the license first (especially if you intend to manufacture or otherwise distribute the device in question). If you are simply building a personal-use "one-off" device, you can pretty much get away with anything (heck, you can even build a patented device in this manner - that's the whole point of a patent, to allow others to build then improve upon such a device); it's only if you intend to manufacture or otherwise distribute to third-parties copies of the device that might get you in trouble, especially if the license on the device/schematic isn't clear. Just do your research first, if this is a goal.