An easy formal definition would be that the digital product is created within a digital format to be used with computers and other electronic devices and is particularly not tangible otherwise.
I do not really like to offer just proper definitions though as they oftentimes offer very little. So originating from a marketing point of view:
A digital technique is a monetisation of your time, know-how and effort. It is how “new money” is made and many folks who do it correctly are becoming incredibly rich from it.
A digital technique is superior to a physical solution in many ways. You only create the item once (you can offer changes though) and it can be marketed to hundreds, thousands or even numerous persons without no control. This ensures a certain higher level of profitability.
You can create the online business/blog around providing digital products.
They can be e books, audio, graphics, whitepapers, program, photographs and much more.