To log in to iCloud you will need to enter your Apple ID and password
You will then be able to see everything you have saved to iCloud.
As the picture above demonstrates you can see all the items you have turned on from your iPhone / iPad which allows them to sync with iCloud.
This is great to know if things are not syncing the way you think they should be.
Knowing how to access iCloud will also let you find a lost device ( iPhone / iPad / iMac )
Need to know more about iCloud , checkout this link from Apple : iCloud Help