Chili flakes are just dried chillies that are then crushed leaving whole seeds and pieces of dried flesh. The heat of the crushed chillies is dependant upon the variety of chili used in the first place. Most crushed chillies from Indian and Asian ethnic supermarkets tend to be from birds eye chillies and therefore tend to be quite hot.
Boukovo or Greek Chili flakes are not usually as hot as normal chili flakes. Because they are oven dried, and sometimes dried over embers, they have a very unique smokey flavour. They are nice as a 'shake over garnish' if you are a chili freak. Try to find a Greek or Turkish food store to get your boukovo. As you can see from the picture, it can be sold in large quantities! This batch was bought back from Cyprus by a good friend.