Several apps let you download existing stacks made by the users of online flash card libraries. Millions of stacks on every subject imaginable. In most apps you can search these libraries on keyword, and some let you browse by category.
Quizlet (5 apps)
5 million stacks of cards. Cards can have pictures. www.quizlet.com
Study Stack (5 apps)
Not clear how many stacks, but definitely enough. Cards don’t have pictures. www.studystack.com
Flashcard Exchange (15 apps)
38 millions cards and counting. Cards can have images, and sounds. www.flashcardexchange.com
ProVoc (2 apps)
Mainly to study languages (stacks are gathered in language pairs). Cards can have images, and sounds. website
Own library (31 apps)
Several apps also have their own shared library for users of the app.
Imports images (43 apps)
Apps that also download the images cards might have.
It can be handy to directly send a stack to anyone who has the same app.
Beam (15 apps)
Can send a stack to another user with the same app on the local wifi network, or over bluetooth. Or, as good as beaming: some apps let you upload a stack to their server and give you a code with which another user can download the stack.
E-mail (39 apps)
Lets you mail a stack to another user. The attachment in Mail app will open as a stack in the flashcard app.
Import & export
Maybe you already have a list or spreadsheet of the things you want to study, or you use desktop program or website that can export the cards as a file. Several apps let you import, and sometimes also export a stack of cards.
Import (97 apps)
Can import a text file in CSV or tab-separated format, or a Google Docs spreadsheet, or provides another way of getting your data from the desktop into the app.
Export (39 apps)
Lets you export in CSV or tab-separated format so you can always move your carefully built-up stack to another application.
When you’re studying a bunch of cards it’s smart to study only the ones you NEED to be reviewing at any given time, and not lose time with the easy ones you already memorized. But today’s ‘easy’ cards might be completely forgotten in a month, so you should probably review them sometime next week, or should you do it the week after that? How to keep track? Enter Spaced Repetition.
Consider this. You make five flashcards for Russian vocabulary words. Russian word on one side, English on the other. Review those cards 20 times a day for a week. At the end of the week, you’ll know those words really well. Now deposit those cards in the bank with instructions to keep them hidden from you for a whole year. Come back one year later and how well you will remember those words? You probably won’t remember them at all. You reviewed each card 140 times, for a total of 700 flashcard reviews, and all for nothing. Pretty discouraging, isn’t it? But what if I told you that you could review each card just 20 times total, and remember them all perfectly at the one year mark? Sound too good to be true? Welcome to the wonderful world of Spaced Repetition Systems. It’s a world psychologists have known about for over a hundred years, but the calculations involved were too difficult to be practical until recently. Science fiction writers have long fantasized about a future where we can quickly learn vast amounts of knowledge using technology. We stand now upon the brink of that future, and that technology is the Spaced Repetition System.
Spaced repetition systems take the spacing effect into account. The fact that we more easily remember or learn items in a list when they are studied a few times over a long period of time, rather than studied repeatedly in a short period of time. (like cramming just before an exam).
In flashcards apps there seem to be roughly 3 different algorithms. We’ll call them Leitner, SRS, and SRS+.
Leitner (38 apps)
The German science journalist Sebastian Leitner used different boxes with cards, as do many people using his system. Each Leitner Box has a proficiency level. Some boxes you review every day, and the level 5 box with the completely memorized cards maybe once a month. Several apps now use this system and put the cards in different digital boxes.
SRS (17 apps)
Stands for ‘Spaced Repetition System’ and is more precise, and impossible to do with real life shoeboxes. Because EVERY SINGLE CARD is tracked. Every time a card is reviewed and answered correctly it gets a new calculated ‘date to be reviewed’ attached to it. And that’s the day it will pop up again in your cards to be reviewed. An example schedule could be: 1 day, 3 days, 7 days, 16 days, 35 days, 70 days, 140 days, 280 days.
SRS+ (12 apps)
You might have more trouble with some cards than with others, so not every card should be set to the same review schedule. Apps with SRS+ also take the difficulty of card into account to make a more precise estimate of the forgetting curve. But this does mean you have to grade how well (or not) you remembered a card. Mostly it’s a choice between just 3 options: ‘Didn’t know’, ‘I knew’ and ‘Almost’. But it can be 4: ‘Didn’t know’, ‘Good’, ‘Easy’ and ‘Very Easy’, or even 5. Many developers use one of the Supermemo algorithms, a flashcard application that has been around since the eighties. There’s an excellent article by Wired on Supermemo and it’s creator Piotr Wozniak.
Flagging (64 apps)
If you don’t want to use spaced repetition you can always flag the cards you memorized so you can exclude them from the stack you’re studying.
No tracking (157 apps)
Shows you every card in a stack. Many apps only have this, but sometimes it’s a nice extra option to have in a more sophisticated app with SRS or Leitner boxes.
Having the same stacks of cards on two devices, or on a desktop program or website so you can add and change cards using a real keyboard.
Sync in the cloud: 36 apps sync with a website or webservice, enabling you to have the same stacks on different iOS devices (or sometimes desktop or other mobile devices) and study on all of them. Most of them can also be used online on their website.
Website version: 29 apps have a website version on which you can add or change cards and also study.
Local sync: 7 apps can sync on the local network. Mostly with a desktop version but sometimes also between different iDevices.
Desktop version: 23 apps have a desktop counterpart running on Mac, Windows or Linux. Some of them sync on the local network, some sync in the cloud, and some can only send stacks to the app but not the other way.
Adding images or sounds to cards, having cards read out by the app, or easier adding of new cards.
Images: 78 apps let you add images to your cards. A bunch of them also imports them from Quizlet or Flashcard Exchange
Image search: 6 apps can search for pictures on Flickr (or similar website) to use on your cards.
Sounds: 43 apps let you record a sound to go with a card. A few of them have this as a main function and let you study by just listening.
Text to Speech: 23 apps can read aloud what’s typed on your cards.
Lookup: 19 apps can lookup a term so you don’t have to type out the other side of a card. They search for a definition and/or a translation of the word you entered. Most of them do this online, but some apps have a built-in dictionary.
Hints: 7 apps have a hint field for each card, in which you can save text to help recall the answer. Viewing the hint is optional.
Third side: 8 apps let cards have an extra answer side: a third side. This can be handy when studying character-based languages, like Chinese or Japanese (card sides for: word, reading & meaning).
Some more functions apps might have.
Type out answer: 30 apps let you answer cards by typing out the answer.
Multiple choice: 29 apps can show you a list of answers you have to choose from.
Text formatting: 10 apps provide formatting options for the text on your cards.
Drawing: 5 apps let you make drawings on your cards.
Scratchpad: 6 apps let you write out the answer before checking it on the answer side.
Latex: 5 apps can format mathematical formulas, using Latex or a similar system.
Reminders: 3 apps can remind you to study regularly.