According to Wikipedia, flashcards are "cards bearing information, as words or numbers, on either or both sides, used in classroom drills or in private study. One writes a question on a card and an answer overleaf. Flashcards can bear vocabulary, historical dates, formulas or any subject matter that can be learned via a question-and-answer format. Flashcards are widely used as a learning drill to aid memorization by way of spaced repetition."
For language learners, flashcards are an excellent way to learn or revise grammar, vocabulary, idioms, and much more besides. And with the availability of web- or app-based flashcard systems, you no longer have to go to the trouble of making actual physical cards.
There are dozens of online flashcard services, but the one I recommend to my own students is Quizlet, which is free and has thousands of flashcard sets created by other users and teachers. I've created dozens of sets myself, and you can access them all here. Many of the sets are English-French, but you can copy and edit sets to include your own translations or definitions. Quizlet also has a range of activities and games which make learning more fun. Although you can use Quizlet on a computer, I suggest you download the app (for iOS or Android) so that you can study on the go using your smartphone or tablet. You can watch a presentation video for Quizlet here.
As I said, Quizlet is just one of many flashcard services and resources available, so here's a roundup of some of the others.
Anki — Only available as a program you install, but has some powerful learning tools.
Brainscape — A web and mobile study platform that helps you learn things faster. Watch video.
Expemo — This app from Linguahouse uses spaced repetition to optimize the learning process.
Free Flashcard Sets (Busy Teacher) — Over 300 sets to download and print out.
Memrise — Create your own courses of rich, multimedia flashcards.
StudyBlue — Crowdsourced learning platform, providing intelligent learning tools including flashcards, notes, study guides and more. Watch video.
Study Stack — Create sets of flashcards and learn them by using various online activities and games. Also available as an app.
Spaced repetition: Never forget vocabulary ever again (Fluent in 3 months)
Flashcard 411: Everything You Ever Needed to Know About Language Learning Flashcards (FluentU)