Difference between Retract solution and Remove solution in SharePoint

In Central Administration if I go to "System Settings > Manage farm solutions" I can see my deployed solutions (via Visual Studio). Clicking on any of my deployed solution takes me to "Solution Properties" which shows option of Retract Solution. Once I have retracted my solution I am presented with another option of Remove Solution. Removing the solution removes the entry from "Manage farm solutions".

I would like to know:

1. What is the difference between Retract Solution and Remove Solution?


2. Why are two options provided by Microsoft?


3. I noticed that after retracting solution my solution folder in TEMPLATE\CONTROLTEMPLATES was empty but it was still there, but my solution folder in TEMPLATE\LAYOUTS was removed completely. Any reason for this behaviour?