The "Usage" tab allows you to check out the state of all your resources on ~okeanos-knossos. It gives a comprehensive view of what you are currently using, plus detailed information about the usage and limits of each of your virtual resources. It is also colored for your viewing pleasure. Green means good, Red means you are hitting your limits :)
Note: The number of CPUs and the size of your RAM refers to the CPUs and RAM used by your active VMs. This means that you can go past your limits, provided that some of your VMs are stopped.