Unity game engine structure

Here are some resources and diagrams to help better explain how unity is structured.

Unity structure
http://www.rivellomultimediaconsulting.com/unity3d-for-game-development/ provides a brief overview of how everything is organised in Unity.

Here is another diagram of the arrangement of components:

https://www.packtpub.com/sites/default/files/Article-Images/1444_01_06.png

The main point to understand is that every game in Unity is made out of GameObjects. Each GameObject contains a set of Components. Each component might have a set of variables for customising it, such as the mass of the object, if a Physics component is added to your object.

You should also glance over the diagrams at http://3dgep.com/scripting-in-unity-3-5/. Most importantly, each GameObject has two distinct opportunities to "do something". The first opportunity is when the object gets created (This is what Start refers to). This is great for setting up the initial state of the object. The second opportunity is called the Update loop. At each logic step of your game, every object gets an opportunity to do something. This is what Update refers to.