Intelligent flash card system for PC, iPhone and Android. The best flash cards I've found so far. If you create an account on AnkiWeb, you can synchronize flashcards between your devices.
If you want to create chinese flash cards, I recommend you to do so using PC version of Anki with Chinese support add-on. You can than synchronize your cards with your phone using AnkiWeb.
Anki is very, very flexible. If you have any problem with your flash cards, Anki most probably has a way how to solve it. Check Anki manual.
Deck is a set of flash cards. You can create Anki decks yourself, you can download shared decks (there are many HSK decks for example in the shared deck repository), or you can download and import my deck:
You can pretty much customize the imported decks. At least, you will probably want to set your 'New cards daily' and 'Review cards daily' limits.