XBox Live "GamerTag" style widget for displaying your Stack Overflow rep.
Log into Stack Overflow, click your name at the top and your id is the number just after /users/ e.g. if your url was http://stackoverflow.com/users/12345 then 12345 is the user id.
The email address is hashed and used for your Gravatar. It isn't used for anything else or stored for any evil purposes.