Dejan Batanjac

Creates meta description write panel for posts pages and custom post types. 
Meta description write panels are not present in the default WordPress installation.
Meta description were traditionally important for website SEO. Today, with the improvement of the software that reads and summarizes the textual documents, they are loosing the importance.
Still you can find them useful on pages with low or even no text.

SEO Meta Description

Creates meta description write panel for posts, pages and custom post types.

Steve

This plugin uses custom fields to allow the page title tag to be customised and differ from the actual page title.
Both meta descriptions and keywords can also be added if required.
Page meta details can be modified on any publicly accessible post type, such as posts and pages, as well as custom post types.
Theme and plugin developers should note that it is possible override the page title within your templates by setting <code>$wppm_title = 'Newly defined title';</code> before the call to <code>get_header();</code>.

Page Meta

Adds the ability to override the page meta title and add in meta descriptions and keywords for pages.

SEO Meta Description vs Page Meta