Theme Question

I am trying to learn WP structure and have made great advances but for the life of me I can not understand why themes have an index.php and usually another called maybe home.php. The code in the index.php does not use a template include for home.php but if you take out some things in home.php they will not show. I tried putting the things I took out in index.php and they would still not work.

Some guru please school me!!!!!!

JIM