Small, encapsulated pieces of Javascript stored as a URL and so can therefore be bookmarked (a.k.a added to the user's favourites).

Bookmarklets are small snippets of Javascript encapsulated in a URL (generally in the form javascript:(function() { var 'Your code here'; }());).

Once a bookmarklet has been saved as a bookmark (a.k.a favourite), it can be executed against the currently loaded web page by simply selecting it.

Bookmarklets can be used for functionally any purpose that Javascript may be used for, but since they're triggered by the user, they can be used to do things not originally intended by a web page's author.

Questions about the design of a bookmarklets on a page (e.g. distinguishing it from a standard, navigable link), or about how to effectively describe installation or usage steps for bookmarklets are appropriate for this tag.