Open Source generally means that the source code for the project is "open", so a user can modify it. Different licenses exist, the GPL is a common one, in which the user is obliged to release their modified source code.
Open Source does not necessarily imply that the product is free, although it often does.
I think you mean the "software" field. In the electrical field "open source" would be a mosfet device that required an external pull down resistor. :tongue:
I've seen some "open source" hardware projects as well (including the Arduino platform which seems to be quite popular here--having never used it, I can't comment on it). Aside from the new name, I don't know if there's really anything new conceptually, since lots of electronics hobbyists and EEs have shared (in magazine, BBS, or forum format) schematics, code, layouts / gerbers, and in turn, refined and re-shared what they had.
Separate names with a comma.