If you use SCCM or have ever had to mess with Configuration Manager on a computer that runs Software Center, then you have seen all of the client actions that you can run. That said, like most of us when we started using SCCM or Configuration Manager we knew we had to run some/all of the actions to make the changes we made in SCCM reflect on the computer we were trying to push something to. After a while, you start to realize that you can get by running just one or two actions instead of all of them. But, using myself as an example, aside from knowing you needed to run them you likely didn’t know what they actually did. For me, it was always something that I was going to “Google” later and figure out what they did at some future time. Well, I recently reached that future point-in-time and thought I’d share the collection (SCCM pun intended) of answers that I found. Hopeful what I’ve complied helps out someone else that finally decided to search about it…

• Application Deployment Evaluation Cycle – This action re-evaluates the requirement rules for all deployments. If an application is required, and not installed when the Application Deployment Evaluation Cycle runs, Configuration Manager automatically triggers a re-install. The Application Deployment Evaluation Cycle only applies to applications and not to the packages. The default value is set to run every 7 days.
• Branch Distribution Point Maintenance Task – Verifies any pre-staged 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 – This 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 the 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.
• Send Unsent State Messages – This tool sends State Messages that are cached on the ConfigMgr client to the ConfigMgr server.
• 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.
• State Message Cache Cleanup – This tool clears State Messages that are cached on the ConfigMgr client.
• 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.

For anyone interested, the descriptions above are not my own. I’ve copied them from here and here.