What are the different types of medals, and how do you get the different medals, particularly gold?

There are 3 types of medals, courage medals, wisdom medals and hero medals.

Hero medals come from recruiting heroes that you already have (duplicates) or from splitting (deleting) heroes that you already have. The colour of the medals depend on the quality of the hero. These are used to unlock skills in heroes or to exchange for wisdom medals.

Wisdom medals come from event rewards (hourly challenges etc) from hero recruitment and a few other sources including from the hero medals above. You use these to upgrade hero skills.

Courage medals come from event rewards and are used for certain research items.