I often hear people talking about adding elements of chance to an experience to make it more interesting. For instance, adding a random reward for doing a certain task in a system.
However, when does adding randomness turn into adding gambling elements? Well, we have to look at the definition of gambling and look at an example.
In the UK the Gambling Commision looks after all things related to gambling and has a long list of things that it considers to be gambling – http://www.gamblingcommission.gov.uk/for-the-public/What-is-gambling.aspx