The Warehouse is a building where you can store, use and purchase Items. The Warehouse has 20 slots by default but you can buy 4 additional slots for 50 Gems. You can repeat this transaction until you have 45 slots, then it costs 100.
The Consumable, Gift Pack and Crests tabs just hide items that do not fall under the selected category. The All tab has every item visible.
The Trade Merits tab takes you to the Merit Store where you can purchase items:
The items available to be purchased are random and as it says, the shop resets every day at 21:00 server time. You can re-roll the available items up to 10 times a day with Gems. The first re-roll per day costs 100, all subsequent re-rolls cost 300. Your Merit supply can be seen in the bottom left hand corner of the screen.
For a complete list of all items and their costs please view the Items page.