What do each of the actions in the SCCM Client actually do?

If you look up the Technet page regarding the Actions tab, you’ll find that it tells you nothing about each action. All of the actions on the tab are scheduled tasks; that is, if the feature is enabled, it will automatically run on a periodic basis. In some circumstances (e.g. troubleshooting), you may find the need to manually initiate these tasks. That’s where this tab comes in. The information on each action’s functionality is scattered about the Technet website:

• Branch Distribution Point Maintenance Task verifies any prestaged packages and downloads any that do not exist on the branch distribution point. While Technet does not explicitly state it, I believe this task is useful only on branch distribution points and is ignored on normal clients.
• Discovery Data Collection Cycle causes the client to generate a new discovery data record (DDR). When the DDR is processed by the site server, Discovery Data Manager adds or updates resource information from the DDR in the site database.
• File Collection Cycle When a file is specified for collection, the Microsoft System Center Configuration Manager 2007 software inventory agent searches for that file when it runs a software inventory scan on each client in the site. If the software inventory client agent finds a file that should be collected, the file is attached to the inventory file and sent to the site server. This action differs from software inventory in that it actually sends the file to the site server, so that it can be later viewed using Resource Explorer. This is a part of SCCM inventory functionality.
• Hardware Inventory Cycle collects information such as available disk space, processor type, and operating system about each computer. This is a part of SCCM inventory functionality.
• Machine Policy Retrieval & Evaluation Cycle The client downloads its policy on a schedule. By default, this value is configured to every 60 minutes and is configured with the option Policy polling interval (minutes). However, there might be occasions when you want to initiate ad-hoc policy retrieval from the client—for example, in a troubleshooting scenario or when testing. This action initiates ad-hoc machine policy retrieval from the client outside its scheduled polling interval.
• Software Inventory Cycle collects software inventory data directly from files (such as .exe files) by inventorying the file header information. Configuration Manager 2007 can also inventory unknown files — files that do not have detailed information in their file headers. This provides a flexible, easy-to-maintain software inventory method. You can also have Configuration Manager 2007 collect copies of files that you specify. Software inventory and collected file information for a client can be viewed using Resource Explorer. This is a part of SCCM inventory functionality.
• Software Metering Usage Report Cycle collects the data that allows you to monitor and client software usage.
• Software Updates Deployment Evaluation Cycle initiates a scan for software updates compliance. Before client computers can scan for software update compliance, the software updates environment must be configured.
• Software Updates Scan Cycle Just after a software update installation completes, a scan is initiated to verify that the update is no longer required and to create a new state message that indicates the update has been installed. When the installation has finished but a restart is necessary, the state will indicate that the client computer is pending a restart.
• User Policy Retrieval & Evaluation Cycle Similar to Machine Policy Retrieval & Evaluation Cycle, but this action initiates ad-hoc user policy retrieval from the client outside its scheduled polling interval.
• Windows Installer Source List Update Cycle causes the Product Source Update Manager to complete a full update cycle. When you install an application using Windows Installer, those Windows Installer applications try to return to the path they were installed from when they need to install new components, repair the application, or update the application. This location is called the Windows Installer source location. Windows Installer Source Location Manager can automatically search Configuration Manager 2007 distribution points for the source files, even if the application was not originally installed from a distribution point.

Source: http://serverfault.com