Stampin' Up! has created two basic types of ink: Craft and Classic. Classic ink is your basic water-based dye ink. Craft ink is different in that it's a pigment ink and thus, more like paint that traditional ink. Because it's made like that, it allows you to do some pretty nifty things. For example, look at the vivid burgundy images on the following scrapbook page:
Copyright Stampin' Up!® 1990-2005
View the page (and many others besides) or leave a comment here, at my gallery: Daddy's Girl.
The stamped letters that spell "daddy" and the three little squares at the top of the page were both stamped with Craft ink. If I had used Classic ink it would not have had the same intensity as you see here. The pigment ink was able to stay on top of the bright cardstock and the color was not absorbed into it. Craft ink is wonderful in that it allows you this flexibility. For example, with craft ink, you would be able to stamp a light yellow image onto dark orange cardstock. If you were to use Classic ink, you'd never see your image!
Also, another great thing about Craft ink for scrapbookers in particular is that it is much less resistant to fading. Since most scrapbookers are looking for products that will last "forever," Craft ink is a great choice for them.
Personally, I like it because, since it's more like a paint, I can use it more often on my scrapbook layouts. For example, I love Classic ink to stamp red flowers to make greeting cards, but if I wanted to stamp flowers all over a piece of Celery cardstock for a background, I'd love to use a White Craft ink instead!
Are you picturing all the possibilities?! Mmm...fun!