Keys 1-4 are run each time a new user logs in.
Keys 5-6 are run in the background when the logon dialog box first appears, or at this stage of the boot process if there is no logon. These keys are for background services such as remote registry service and are run only once per boot.
Key 7 is run as part of Setup's first-boot activities, or after you use the Add/Remove Programs Wizard.
Under each of these keys is a series of subkeys. The subkeys are used to allow multiple sub-entries to exist without overwriting one another.
The value for a subkey is a command line. By default, Run keys are ignored in Safe mode. For keys 3-4, the command line can be prefixed with an exclamation point to defer deletion of the subkey until after the command has been completed.
For keys 3, 4, and 6, the subkey is deleted before the command line is run unless overridden as noted above. As a result, if a RunOnce operation fails to run properly, the component that failed will not be asked to run again the next time you start the computer.
Key 7 is used only by Setup. This key displays the progress dialog box as the keys are run one at a time. For key 7, the name of the subkey is the name that is displayed in the dialog box.