Lesson 5 discusses the different ways to access a document stored within a document library. One way is to simply click the document name within the library, which will open it in read-only mode. If the drop-down menu is accessed by hovering the mouse over the document name, the user gets the option to edit in Microsoft Office Word (assuming Word is installed on the workstation), which opens Word and allows the user to work on the document normally.
SharePoint is also designed to allow documents to be "checked out," which locks the file in SharePoint so no one else can edit it. Best practice is for users to check out a document before they click the Edit in Microsoft Office Word option. This locks the file and also lets other document library users know that the document is already checked out as well as lists who has the document checked out.
If the document has been checked out and changes are made to it, the user has the ability to check the document back in when he or she closes it, or the user can select Check In from the File menu in Word 2003. This opens the Check In Comments window, where comments can be entered for that version of the document. If document versioning is turned on for the document library (see Lesson 12, "Alerts and Document Versioning"), these comments can be viewed from the Versions page (Version.aspx) for that document.