Differences between CS Photoshop and CS illustrator

I've been using CS Photoshop for editing pictures and making logo banners. I found this application as the most user friendly when it comes to editing photos and making 3d logos and stuff. But I've heard that CS Illustrator is also a good application that you can use to make great banners and 3rd logos. I might consider using CS Illustrator in the future but can someone tell me what are the difference of the two applications.
Thanks.
