iCloud can't be used to free up space on your device. Even though it hosts your synced data on the iCloud server, a local copy is always kept on your device so you have access to it even when you don't have a connection to the internet (such as when you're on a plane or in an area without internet connectivity). You can't have photos only in iCloud. A local copy of your photos will always remain on your device. They are only streamed to the server temporarily (30 days), allowing enough time for your other devices to connect and receive them, at which time a local copy is stored on your other devices as well.
There also isn't a "directory" of what is stored in iCloud. You can, of course, see all of your synced data on your device from within the apps that are syncing data with iCloud (contacts, calendars, etc.). You can also see categories of your iCloud data by going to Settings>iCloud>Storage & Backup>Manage Storage.
icloud is image based backup. That is what you are paying for. What is inside of image file you can find here
You can't pull information out of image file, but you can restore that image to another device in case if your iphone/ipad becomes defective or lost/stolen.