Everything you needed to know about Radio Receivers and Scanners

Tuning into the airwaves for new and interesting transmissions has fascinated enthusiasts since the very earliest days of radio. As a result of advances in computer and radio technology, the equipment packed radio rooms of the past are now replaced by scanning receivers sometimes just the size of a mobile phone.
What can radio receivers/scanners pick up? What types of radio receivers/scanners are there? What can and cannot be monitored legally? Best practice when using a radio receiver/scanner