A Bookmark is an object used to identify the location of something. The most common usage of a bookmark (and where the word originates) is in use with Books, for example marking the latest page of a book which one has read up to with a bookmark (otherwise known as bookmarking).
Bookmarking the current page one has the result of allowing one to easily start up reading exactly where one left off (i.e. Johnny bookmarked his page to "Chapter 3 - Pg. 234"). Bookmarking could be even more detailed in that a specific paragraph of a page could be bookmarked (for example, using a highlighter, pen note, piece of tape, post-it note or other identifier to specify how far in the page one has read).
Bookmarking on the Web
On the web, bookmarking was first used to describe the process of bookmarking a web page or website within a web browser. (i.e. by pushing a shortcut command such as CTRL+D, or, using the browser's menu "Bookmarks-->Bookmark This Page" as in FireFox).
Social Bookmarking is a new spin on typical browser-based bookmarking, where bookmarks reside within 3rd party Web Services' networks and server infrastructure not just within a single computer's local hard drive. Services such as Digg and Delicious were the first Social Bookmarking sites to reach widespread appeal, and largely influenced future services.