DS lite is smaller and subsequently a little more expemsive. All games should work on both however there were some minor updates on the lite. The only thing i can see that works on the lite and not on the ordinary ds is that you can browse the web on the lite (assuming you've bought the right software).
Just to add to what kitandholly said the screen on the DS lite is much brighter, has better colour rendition and can be viewed at a wider angle than the older screens. Overall the DS lite is a better design, more stylish to look at and feels less plasticy, the stylus is a better design too, far less likely to fall out of it's slot.