I want something to show in sidebar only if it is not on the homepage..

Hi I hope this makes sense.. I want something to show in the sidebar only if it is not on the homepage. I can make it show no problem(by include it.php) .I expect I need to use an if/else command but what code do I type in so that it decides whether or not it is the homepage?..
And sorry what is a codex and where can i find it?