Try to explain briefly what a command is at the top

Extending/Commands.md

@@ -3,6 +3,12 @@
 
 # Commands
 
+# What is a "command"?
+
+A "command" looks similar to a task: it's a named operation that can be executed from the sbt console.
+
+However, a command's implementation takes as its parameter the entire state of the build (represented by [State]) and computes a new [State]. This means that a command can look at or modify other sbt settings, for example. Typically, you would resort to a command when you need to do something that's impossible in a regular task.
+
 # Introduction
 
 There are three main aspects to commands: