Mnemosyne uses a sophisticated algorithm to schedule the best time for a card to come up for review. Difficult cards that you tend to forget quickly will be scheduled more often, while Mnemosyne wont waste your time on things you remember well.
If you want, anonymous statistics on your learning process can be uploaded to a central server for analysis. This data will be valuable to study the behaviour of our memory over a very long time period. The results will be used to improve the scheduling algorithms behind the software even further.