The hallmark of OSS is that you can access a program's source files. Source files are the files that programmers create when they write a program. These files are written using a computer language such as Java or C or Basic. They are then translated into instructions a computer can understand and the program that does this translation is called a compiler. The translated files that the computer runs are often referred to as binary files or binaries.
Most commercial software is released as a binary file and the source file is kept secret.
One of the advantages of being able to access the source files is that you can then alter the program so that you can make it do something different or something new. Many GPs have actually written their own medical software to suit their needs. However, programming is not for everyone. One advantage of OSS for non-programmers is that they can pay a programmer to write the added features they want. This can be of advantage to groups who want to have the same additional features, such as GPs involved in divisional projects.