Anki is another good app. It is free on Android, $20 on iPhone, free on the PC, free on the web, and all of them sync to each other (the iOS one is more or less only pay because the community begged the author, who has been making the app for free for over 20 years, to allow them to pay as a thank you). It is maybe the best flashcard app around, based on the spaced repetition idea.