The Inventory page displays all the items you've obtained in the game, including items earned as rewards while completing missions, purchased items, gifts from friends, and so on.
You can access your inventory by clicking the 'Inventory' link in the main menu.
All the items you have are split into groups according to their type and purpose. You can switch between groups by clicking the group name at the left side of the item list, items from the selected group will be displayed at the right side. If you don't have items from a certain group - the group name won't be displayed.