Alert Types
There are many types of alerts that you can receive based on different actions performed around this site. These are the current different actions that cause an alert to be received:

Reputation

This alert type is received when somebody modifies your reputation.

Private Message

This alert type is received when somebody sends you a new Private Message via the site. It will show you the sender and the title, much like the notice at the top of the site.

Added to Buddies

This alert type is received whenever another member of the site adds you to his/her buddy list.

Quoted in a post

This alert type is received whenever another member of the site quotes you within a post anywhere on the site.

New reply to your thread

This alert type is received whenever another member of the site replies to one of your threads. This alert is only received for the first new post to the thread rather than being duplicated for each reply.