There are lots of very helpful sites out there, but let me give you a perverted version, skewed to a stamper's perspective.
Art journalling, for me, is a spiral bound sketchbook that I use to jot down layouts I come across while waiting at the doctors office and flipping through magazines. I also use it to practice my copic coloring skills, and recording color blends that I want to make sure to remember. I use it to record bits of my daily life, and add some decoration - perfectly suited to Gina's stamp lines! The list goes on and on.
I also use pages for little trial applications. I practice using new inks, test new techniques (like shaving cream backgrounds, alcohol ink smearing backgrounds, etc.). As a result of researching art journals, I've found a LOT of non-traditional art products and techniques that work wonderfully in traditional stamping applications.
The biggest attraction, and frankly what makes me addicted to this, is the freedom to PLAY without worrying that I'll destroy my best products. I use scraps and bits and sayings and stories that appeal to me at the moment. The time investment can be as small or large as you have space for. The products are what you have on hand. In a nutshell:
PLAY. SHARE. LAUGH. CREATE!