Rollback returns a file to a previous version, erasing all the versions and changes since then. All changes made after the version rolled back to are lost. For example, if you click on version 5 and then click Rollback, all changes since version 5 are lost.
If the file you roll back to is being shared by more than one project, the rollback affects only the project you specify (the current project, by default). To accomplish this, rollback performs an implicit branch, that is, it branches the file in the specified project away from the same file in all other projects, and creates a new development path. To bring the disparate development paths back together, if you make changes to the specified project, you have to merge the files.
Note You can also perform a Virtual Rollback, which doesn't erase all versions and changes of a file. This is the preferred method for performing a rollback.
For information on how to rollback to previous versions in VSS, refer to: