As the name implies, Twitterank is sort of like Page Rank for twitter users. True to its namesake, it uses "back references" of sorts to determine how worthy of a person you are in Twitterverse. Read the FAQ for more info.
I'm not out to steal ur twitterz. Frankly, I wish I didn't have to ask for your account info, but Twitter doesn't offer APIs using any other authentication mechanism (according to the docs). Read more about what I will and won't do with your account info/data in the FAQ.
I will not store your password. I will only use it once to calculate your Twitterank.