To conserve disk space, Time Machine excludes several directories from its backup that are used for temporary files or cache files. Since these are not critical files—and they aren't meant to be preserved indefinitely by the operating system—there is no reason to include them in a backup. You can also choose to exclude additional items from a backup.
Clicking the Options button in the Time Machine preferences displays a dialog box that includes a "Do not back up" list. You can add items (files, folders, and even hard drives or partitions) to this list by dragging them into the listbox from the Finder or by clicking the Add button beneath the listbox.
If you change your mind and want to include an item, simply select it and click the Remove button. The hard drive used by Time Machine to store backups is automatically included in this list.
Ideally, you will want to back up your entire computer. However, if you have a small backup drive, you might need to limit backups to only important folders. You might also choose to exclude folders that third-party applications use for storing temporary files (though most adhere to Apple's guidelines for these files and should be excluded automatically).