What is an API key?
The API key is used by Bungie to measure who and what is using the API.
How do I get one?
Visit the developer portal on bungie.net.
How do I use it?
In a HTTP request, include the following header:
Is the key mandatory?
Yes, although as of May, 2015, it is not being enforced. However, you should include one now to avoid your application breaking in the future.