In Drupal, user_load($uid) function will return an user object. And Drupal has defined a global user object as well (it represents current logged in user).
The user object has following properties:
status -> active/blocked
uid -> user id
mail -> email address
created -> created timestamp
login -> login timestamp
hostname -> ip address
init -> the email address provided at initial registration
roles -> roles assigned to this user
The difference between “mail” property and “init” property is that mail can be changed after registration, but init is the first used email address and cannot be changed. If you want to send email to an user, you need to know his email address ($user->mail).