Focus groups assess what users say: a number of people gather in order to discuss their feelings, attitudes and thoughts on a given topic to reveal their motivations and preferences.

Usability testing, on the other hand, is about observing how people actually use a product, by assigning key tasks to users and analyzing their performance and experience.

https://uxmyths.com/post/1319999199/myth-26-usability-testing-focus-groups

Busted!