When you touch a lorestone, you'll be told a bit of lore and be given a small amount of XP. There is also an achievement for activating all of the lorestones. The game keeps track of which ones you've activated under the Status section:
Each found set will also grant you a permanent bonus. A small check mark will appear next to the lorestone set once you've found all of them to show that you've gotten that reward.
For example, in the picture above, I found all the lorestones in the Castle Windmere area so I received a permanent 2% increase to critical hit.