It’s a bit like riding a bike - well almost
Open source software (OSS) was first introduced in the 1970s by Richard Stallman, an American software developer. Richard believes sharing source-code and ideas is fundamental to freedom of speech. He developed a ‘free’ version of the widely used ‘Unix’ operating system and this program was released under a specially created General Public Licence (‘GNU GPL’). This licence ensures that the source-code remains available to all and allows commercial usage or distribution. Richard didn't mean that 'free' meant no cost rather that it meant free to modify. There was confusion and to avoid this, the term open source came into play.
Proprietary or closed software belongs to an individual or company and this software’s source code — the bits that programmers write — is not made available. In open source every part of the software can be seen which allows users and developers to modify and improve it, tailor it and tweak it and to distribute any changes that are made.
Both OSS and proprietary software companies make money. Companies such as Microsoft or Oracle develop closed source software and then sell licences to use it — which may be a percentage cost of every computer sold, or costed by the number of ‘seats’ that an organisation purchases.
OSS companies make money by providing services. These may be advising clients on the software which provides the best solution to their problems, writing and installing customised software, further developing an application and software maintenance. Cambridge Open Systems fits into this category of company, providing solutions for its clients.