So the first, and perhaps easiest to explain is what we call the "Starred Words" system. As you are practicing, it allows you to mark words for later, more focused review. Starring words does not affect how Skritter grades them, it simply allows you to better learn words or characters that are giving you problems.
You have to star several words before you can use the feature otherwise the list isn't long enough for Skritter to schedule anything. Once you've marked a few, you'll see a "Starred Words" section appear at the bottom of the Practice Nav that allows you to study just those words.
I have been helping to test the system for a little while and think it's going to really help me pull apart those pesky homonyms that I have so much trouble with!