"Kateřina a můj syn jsou přátelé."

Translation: Kateřina and my son are friends.

[Jelena482934]

Is přátele nominativ plural?

[VladaFu]

yes, you can often just check Wiktionary yourself:

https://en.wiktionary.org/wiki/p%C5%99%C3%A1tel%C3%A9

https://en.wiktionary.org/wiki/p%C5%99%C3%ADtel#Declension

[dudeitszack]

Is there a difference between kamaradí and přatelé?

[VladaFu]

Přátelé is more formal, kamarádi is less.

In singular it is more complicated as přítel/přítelkyně can also mean boyfriend/girlfriend.

[pdusek]

Normally, especially in plural, I would translate "kamarádi" as "friends" and "přátelé" as "very good friends" (although the series "Friends" was translated as "Přátelé".). I agree with VladaFu that přítel is more formal.
But if used in singular, as Vlada mentioned too, it means almost always a boyfriend (přítelkyně would be girlfriend), used in informal language for almost all non-married people.