Open Source software has created a revolution in how technology can be acquired and used to improve your businesses bottom line. One ‘official’ definition is located here http://en.wikipedia.org/wiki/Open_source_software#Open_Source_Definition .
I would describe Open Source software as a movement. Developers organize and volunteer to create software products (websites for example) which can be used for free by the general public. ( There are limitations, but the overview is that it is free to download and use ) . The community of developers and users work together (usually) to create a list of bugs which need fixing and enhancements to the product. The larger the community (and by definition the more a product is used), the more likely that new, relevant features will be requested and built into a product in a reasonable time frame.
This creates a unique dynamic in the technology marketplace. Instead of having to purchase software and pay annual maintenance fees, products can be built with neither of these costs. This can be make a huge difference to the bottom line of any company.