Smart - only emails you once per item on the forum until you've viewed the item you're being notified about. So if you were notified about a thread you wouldn't receive any further updates about that thread until you visited the forum and viewed the thread.
Instant - emails you every time a notification is triggered.
Daily - only emails you once per day for all of the day's notifications.
Weekly - only emails you once per week of all the week's notifications. A week is Monday thru Sunday.