A board is the place where your groups of pulses are held. It's a virtual white board where projects are stored as a way to keep track of everything from everyday tasks to month/year long projects... but so much better!
Here's an example of a board with several projects, the person in charge and the stages they go through from start to finish :)
And on the right hand side, you will see the drop down gear wheel where various options are located including where to add a new group of pulses.
A folder is simply a "storage" space for boards. Click the "+ New" sign on the side bar, select New Folder, and label it with a New Folder Name. You can then drag and drop other boards into that folder.
This is a great way to keep alike boards together. This is also useful for different departments in a company by creating folders such as: sales, marketing, design, R&D folders, and then drag and drop boards that are related to those labels :)
Here is an example of a folder: