What is the difference between Activity and Context?

Context is the Base Object. So every Activity same as Application derives from Context. This means that every Activity and every Application IS a Context.

An Application context lasts, as long as your app is alive, while the Activity context dies with your Activity (it is not valid after onDestroy of that Activity).

So if you need the Context across Activities (i.e. in a Singleton) you will be better off using an Application context.