What is the difference between fast scan and full scan?

The fast scan scans all installed applications (apps) . The full scan additionally scans the content of the SD-card or other built-in memory, where your personal images, videos and music are stored.
Some apps might download a malicious html file, which is invoked to bill the user from time to time. Since the file is downloaded, it will not be found by a fast scan, due to that it’s placed on storage and might not be in the memory at a given moment
The real time protection will find it if ON, and if its OFF, it will be found when performing a full scan