What are ‘NOODP’ and ‘NOYDIR’ Meta Tags and why they are used?

[Kalpart]

NOODP - No Open Directory Project
NOYDIR - No Yahoo! Directory

If someone types in a keyword into the search, Google may use descriptions from the open directory project or from Yahoo! directory as the page title and meta description for your organic listing. The NOODP tag allows you to essentially opt out of the open directory project title and description override. And NOYDIR tag allows you to essentially opt out of the Yahoo! directory title and description override