When you create a script and and attach it to a GameObject, the script appears in the GameObject's Inspector just like a Component. This is because scripts become Components when they are saved - a script is just a specific type of Component. In technical terms, a script compiles as a type of Component, and is treated like any other Component by the Unity engine. So basically, a script is a Component that you are creating yourself. You will define its members to be exposed in the Inspector, and it will execute whatever functionality you've written.
Read more about creating and using scripts on the Scripting42 page.
Page last updated: 2013-07-31