Hmmm, that´s a long story to tell.
The main difference between is:
Illustrator is for drawing scalable VECTOR graphics. Images are created in Illustator by using vectors, lines, nods.
Photoshop is for editing color, brightness, hue, of a photograph or BITMAP picture. You can also add filters, textures and other cool effects. Images edited in Photoshop are made of PIXELS.
Sorry about my english. Please feel free to correct me.