In a nutshell TweetMeme is a service which aggregates all the popular links on Twitter to determine which links are popular by categorizing them into Categories, Subcategories and Channels. Using the TweetMeme Button you can discover new content and tweet about it.
What is a shortcode?
A shortcode is a WordPress-specific code that lets you do nifty things with very little effort. Shortcodes can embed files or create objects that would normally require lots of complicated, ugly code in just one line. Shortcode = shortcut. — Shortcodes