Hardware is a general term for the physical artefacts of a technology. It is commonly used for the physical components of a computer system (computer hardware).
What is Free Hardware
Free Hardware consists of physical artefacts of technology designed and offered in the same manner as free and open source software (FOSS)
- Information about the hardware is easily discerned and shared.
- Hardware designs are all released in a similar manner to FOSS.
- The software that drives the hardware is also released as FOSS.
"Free software is often available for zero price, since it often costs you nothing to make your own copy. Thus the tendency to confuse free with gratis. For hardware, the difference between free and gratis is more clear-cut; you can't download hardware through the net, and we don't have automatic copiers for hardware. (Maybe nanotechnology will provide that capability.) So you must expect that making fresh a copy of some hardware will cost you, even if the hardware or design is free. The parts will cost money, and only a very good friend is likely to make circuit boards or solder wires and chips for you as a favour." - Richard Stallmann 
It is quite similar to the term free hardware, some people consider it a more precise definition.
"Firmware such as programs for programmable logic devices or microcoded machines are software, and can be copylefted like any other software. For actual circuits, though, the matter is more complex. Circuits cannot be copylefted because they cannot be copyrighted. Definitions of circuits written in HDL (hardware definition languages) can be copylefted, but the copyleft covers only the expression of the definition, not the circuit itself. Likewise, a drawing or layout of a circuit can be copylefted, but this only covers the drawing or layout, not the circuit itself. What this means is that anyone can legally draw the same circuit topology in a different-looking way, or write a different HDL definition which produces the same circuit. Thus, the strength of copyleft when applied to circuits is limited. However, copylefting HDL definitions and printed circuit layouts may do some good nonetheless." - Richard Stallmann 
Open Hardware Difference
The distinction between copyleft hardware and open source hardware is very similar to the distinction between Free Software and Open Source Software. Copyleft hardware is essentially requiring that all plans for Hardware design (i.e.schematics, bill of materials and PCB layout data) are released under the Creative Commons Attribution-ShareAlike license and that the software needed to both manufacture the device and at least some software, including drivers, necessary to use the hardware is released under the GNU Public License. Technology for copyleft hardware are to be patent-free.
Here are some examples of Free Hardware / Copyleft Hardware projects listed:
- Arduino and Freeduino - are free and open-source electronics prototyping platforms based on flexible, easy-to-use hardware and software
- FreedomBox - is a community project to develop, design and promote personal servers running Free Software...
- RBOS3D - Development of a first generation copyleft 3D printer.
- RepRap - is a Free/Open Source 3D Printer