Mouseware uses a cryptographically secure random number generator based on your mouse movements to generate secure, memorable passwords. Passwords are generated entirely in the browser, no data is ever sent over the network. The generated passphrases are similar to those generated by Diceware or popularized by xkcd, with an emphasis on easy memorization.
1000 guesses/second is a worst-case web-based attack. Typically this is the only type of attack feasible against a secure website.
100 billion guesses/second is a worst-case offline attack, when a hashed password database is stolen by someone with nontrivial technical and financial resources.