For a successful login each day with the same character, you will get interesting rewards just for showing up.
The more days you can keep the "login chain" whole, the better the reward will be.
As the rewards vary greatly so a list of specific items is not provided. Generally there is a wide range of valuable items from Equipment Essences and Random Gems, to Mounts or even Grade A Cards. Items get more valuable if you keep logging in. If you "miss a day, the count resets" and the items you get to collect will be back to the least valuable ones. Recently another reset point (other than missing a day) has been added: once a month, the accumulated days will reset.
You can always see your actual rewards and other, "coming soon", ones. Once you have claimed your login reward for that day, you are able to see the "coming soon" rewards that replaced those that you just collected.
Claiming the RewardsEdit
Press the Login Rewards icon, in the newly opened window press the bookmark named Login Reward and then press the Collect button.