If you reread chapters of a book in intervals, you are doing spaced repetition. If you return back to previous topic in math and do exercises again, you do spaced repetition.
Flashcard are just easy to implement application of it.
Yes among flashcards, anki is the most popular. But naive use of it leads to a lot of pain and tons of problems, you have to learn how to use anki.
I think it is pretty well established that simply re-reading can trick you into thinking that you know the material (via recognition) instead of actually remembering/understanding.
