Working With the Bulk Mail Folder
The Bulk folder is a special, permanent folder for storing messages that SpamGuard identifies as spam. The Bulk folder doesn't generate spam, nor should you receive any additional mail because of this folder.
You can control the deletion of messages in the Bulk folder. In addition to specifying the amount of time messages should remain in the folder before Yahoo! Mail automatically deletes them, you can also delete messages manually.
This is what yahoo! people say. Actually bulk mail contains those mail items that the server thinks are spam kinda. Ther are usually seperated from the important mails automatically1