Apple ID password vs iCloud password
What is the difference between an Apple ID password and the iCloud password. My Mac, iPad and iPhone seem to require both at different times (as well as a DIFFERENT login to access the computer and DIFFERENT passcode to use the mobile devices). I'd sure like to simplify, but can't figure out how to reset the iCloud password using an email address that I have access to. Help!!!
iMac, OS X Yosemite (10.10.5)