Extensions and Bookmark sync. features are now available with Chrome, previously these are available to beta version of chrome only. From 25th Jan onwards these are available to the standard version of Chrome.
These Extensions lets you add more functionalities to your browser like adding Social network buttons, Online shoping, news sources and other stuff. There are over 1500 extensions are available for you at extension gallery. On visiting the extension gallery it will warn you to choose the standard version of Chrome if you are using the beta version.Bookmark sync will be useful to you if you have one or more PC's or laptops( most of the people worked on several computers and on laptops in Office or at home) this feature is very very usefull in such cases. By enabling this feature you can access your bookmarks from office or from home or from any other work location. so you need not create bookmarks manually. watch this video published at youtube to learn how to enable Bookmarksync.
Alternatively you can access extensions from your browser (Settings> Estensions.)
Or you can find at the bottom of the new tab page.