README Effect and Aff

README.md

@@ -26,6 +26,17 @@ main = launchAff_ do
   log response.body
 ```
 
+## Effect and Aff
+
+`Effect` is a synchronous effect monad. It is used to sequence effectful foreign (JavaScript) code — things like random number generation, reading and writing mutable values, writing to the console and throwing and catching exceptions.[<sup>1</sup>](https://stackoverflow.com/questions/37661391/what-are-eff-and-aff)
+
+`Aff` is an asynchronous effect monad. It can handle and sequence effectful asynchronous code, like HTTP requests, timeouts, and network and file IO. 
+It also provides a nice mechanism for handling errors.
+
+Furthermore, `Aff` can perform synchronous effects by using `liftEffect`. In other words, `Effect` is a special case of `Aff`, for the special case that we expect the effect to complete immediately.
+
+Many I/O-related packages in the PureScript ecosystem provide both an asynchronous callback-based `Effect` API and an `Aff` API for the same feature. When you encounter this, you should almost always prefer the `Aff` API. You will find the `Aff` API is much simpler to use correctly. 
+
 ## Documentation
 
 `aff` documentation is stored in a few places: