iCloud is automatic if enabled, and works with a single iCloud account, giving access to all photos from every iDevice that's logged into that account. The devices can be anywhere with internet connectivity. iCloud can be switched on or off from Settings.
AirDrop allows you to select individual photos or files and send them to a nearby iDevice, within Bluetooth range. You can select people in your contacts, or everyone nearby with AirDrop. AirDrop can be switched between off, contacts, or everyone in the Control Center. Control Center is reached by swiping up from the bottom of the screen.