First- for anyone wondering what Anki is- Anki is a free program that you download onto your computer and it lets you create flashcards, import images into them (really useful- I did this with the images in Feralis' notes when studying for the DAT), import media/audio/video into them, and it brings up the flashcards on a frequency based on how often you get them right. (This is the key thing about Anki).
So if you get a flashcard right the first time, it'll bring it up in 4 days. In 4 days, if you get it right again, then it'll bring it up in 12 days (the interval increases if you get the card consistently right), etc. etc. You can adjust the interval for your own personal comfort. Meanwhile, if you get a flashcard wrong, it will bring it up the next day and will keep doing so until you get it right.
The idea is to save your time on the cards giving you trouble rather than wasting time repeating cards that you already know. However, every so often, by repeating the cards you already know, you are ensuring you don't forget that material.
This feature really shines when you have BIG card decks...like 200+.
I used to be a big Quizlet fan. They have a "star" cards feature which you can use to highlight challenging cards too, but to me, Anki is just much more useful once you learn how to use it because of additional features that Quizlet lacks. Especially with the importing of the media- With Quizlet, you can't do all those functions unless you pay extra. With Anki, it is free. And those functions are very important for visual learners like me.
Example: I am having trouble memorizing how chemiosmosis in the mitochondrion works. Well, I just google an image of Chemiosmosis and import it into my Anki flashcard for it, and now I'll see the image every time it comes up and learn it better (I'm a visual learner).
And Quizlet is not a learning program like Anki is.
I'm really happy I made the switch to Anki, and I am never going back to Quizlet.