A board represents the processes in your Company, such as Development, Sales & Marketing, HR and so on. 

A project unites boards related to one product. 

For example, you are developing a travel application. Your Company consists of 4 departments: Design, Development, QA & Support, Marketing & Sales. Your Project is called Travel App and you have the following boards: Travel App Web Development, Travel App Mobile Development, Travel App Product Ideas, Travel App Marketing Strategies and so on. The main thing is, all the boards belong to a single product = project. 

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