There are two main elements to the GPL, designed to prevent open source software being exploited by their inclusion within non-free, closed sourced software products:
- Software that includes source code licensed under the GPL inherits the GPL licence.
- If compiled binaries of software licensed under GPL are distributed, the source code must also be made available by the distributor.
These aspects of the licence discourage exploitation, because if a closed sourced software product that includes open source software is sold for a fee, anyone purchasing the product could demand the source code and redistribute it for free.
Apart from this, the licence is designed to offer freedom, in particular it does not force users of the software to make modifications or developments publicly available. That means that software such as OpenFOAM can be used as the basis of in-house, proprietary software.