Create passwords up to 50 characters long from selected sets of characters.
Select the character sets you want considered when the program is selecting characters.
Suppose you selected characters to include lowercase letters, uppercase letters and digits.
If you Select Evenly from: All Chosen Characters, all 26+26+10 characters will make up one set of 62 characters and each random selection will be made from this full single set.
If you Select Evenly from: Each Chosen Set, all characters will be selected as evenly as possible in rotation from one set at a time. For example, an 8 character password will be randomly chosen in order from lowercase, uppercase, digits, lowercase, uppercase, digits, lowercase then finally uppercase. This selected password set will be shuffled.
http://www.cs.umd.edu/faq/Passwords.shtml
https://accounts.google.com/b/0/PasswordHelp