When you have downloaded the Mendeley Web Extension, you will be able to save references anywhere on the Web. Whenever you have a resource you want to save, all you need to do is click the Medeley button that was added to your toolbar. A window will popup on the side of your web browser and the article information should auto-populate.
Once all the information has been entered in (even though Mendeley will auto-populate a lot of the information, sometimes it might miss something, always double check if you need to add/edit any information!), you click on the Save button, and the article will be added to your web library. 
If you downloaded Mendeley Desktop, everything you add to your Web Library (through the Medeley Web Extenstion) will synch to the desktop version. There is a quick tour Mendeley offers the first time you use the desktop version.
Once your articles are synched from the web extension, you will have the option to add the files to folders you create, you can create a group that you can share your sources with, you will be able to export the files to MS Word, as well as find related material with the "Related" button.
You can change the view of your library from "List View" to "Citation View" as well as change the citation style in the View tab on the menu bar. 