When you want to modify a document, you either check it out of the database, or tell VSS you want to edit the file. VSS copies the file from the database into your working folder. You can then modify the file. If anyone else attempts to check out the same file for editing, VSS generates a message stating the file is already checked out. This simple checkout protocol ensures that conflicts do not arise among multiple users working on the same file. If desired, you can change the project properties to allow multiple checkouts of the same file to different users. Note that binary files are never allowed to be checked out by multiple users.
After you are finished editing the document, you check it in to VSS. This copies the modified document from your folder into the VSS database, making your changes accessible to other users. VSS stores all the changes that have been made to the document; the most recent copy is always available, but earlier versions can be retrieved as well. The VSS reverse delta technology ensures that all versions of a document are available, but uses a minimum of disk space.
If you've made no changes to the file, you can also undo your checkout. The file returns to the way it was before you checked it out.
If you want to just read a file, and don't need to edit it, you can either Get the file into your working folder, or use the View File command to get the most recent version of the document.