Primarily it's a backup tool which will synchronise folders. It also has a command line interface and will use variables in the destination folder name to generate regular sets of backups. (See the examples in the help file included with the software.)
You can use the ySync user interface to pick source and destination folders, then click the Copy or Sync button. If you click Copy all new and changed files and folders will be copied into the destination folder. If you choose Sync the program will do another pass afterwards to remove stale files - those which are no longer present in the source folder. Using Sync, you will end up with an exact duplicate of the source folder. (If you don't know why that's a desirable feature, you don't need ySync - just use XCopy.)
ySync is even more useful as a command line tool. You can specify destination folders using variables (e.g. %DAY% for the day of the week, %YEAR% for the year), allowing you to keep incremental backups. For example, I have a system task which backs up My Documents to a series of folders MON through SUN, giving me seven complete snapshots. If I want to back track to an older file, I simply go back through the folders until I find an earlier version. I also burn the folders to DVD from time to time, giving me a complete weekly snapshot.

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