Fullscan = Scans your computer for viruses and malware threats
Quickscan = same as above but only covers those area activly running malware is known to hide

https://www.avast.com/faq.php?article=AVKB177#artTitle

Smartscan does the same and then some https://www.avast.com/faq.php?article=AVKB159#artTitle

There is also a screensaver scan, that start and stop with the screensaver and most important realtime protection, all AV have this (exept ClamWin) meaning evrything that goes on in your computer when on is checked

there is lots of info in FAQ section … for those who look https://www.avast.com/faq.php?

also videos https://www.avast.com/faq.php?q=video