The Sieve language

The Sieve mail filtering language (RFC 5228) is a simple language that is optimised to allow efficient server-side filtering of mails. A sieve mail filtering script can be installed from the user and is executed on the mail server at delivery of the mail to the user mailbox. Sieve is a modern alternative to the cryptic language of procmail filter scripts.

Managesieve

In modern mail systems the user has no longer file system access to any directory on the imap mail server. Therefore the user must be able to remotely manage his serverside sieve script. Managesieve is the solution to this task. It's a management protocol specification over which the sieve script can be manipulated remotely over the network with a service running on the imap server. All sieve plugins and client sieve editor programs are based on the Managesieve protocol.