Difference between 'api' and 'implementation' in android studio 3.0 gradle dependencies

Since android studio 3.0 when I build a project a new syntax implementation is by default added in gradle dependency section like below:

implementation 'com.android.support:appcompat-v7:26.1.0'

We can also use api instead of implementation here like below:

api 'com.android.support:appcompat-v7:26.1.0'

What is actual difference between implementation and api really? when we should use what as usual?