There are currently four different types of landmarks. Contribution, 20 per: gives rubies. Experience, 2500 per: gives coins. Max rating, 50 per: gives boxes. Mastery 5 per. Each give better and better items after you hit 5, 25, 125, and 625. These can be found on the menu by clicking the lower left box. titled "View Landmarks". Landmarks will show you exactly how far the next one is, so no need to keep count.
Landmarks are great ways to progress faster as once you get higher rating/experience the landmarks give a ton of coins, rubies or even a rare/epic piece (mattering on the tier of landmark).
How landmarks work Edit
Whenever you reach a landmark (for an example 7500 exp) a small ! will appear on the corner of the landmarks button. This shows you can claim a bonus of rubies, coins or a box, depending on what you can get.
When you are able to claim a landmark the bar will be full and a button will be lit up, showing you exactly which one you can claim. On the right is an image showing you can claim a coin bonus for my experience.
WORK IN PROGRESS
Rewards are how much you get from each section of landmark.
Although i do not know all the values here are my estimates.