When your computer is running sluggishly, it can be helpful to cull the number of programs you have open. This includes cutting down on background processes which run invisibly, sometimes without your knowledge.
The best place to start when monitoring apps is the Task Manager. Launch it from the Start menu or with the Ctrl+Shift+Esc keyboard shortcut. You'll land on the Processes screen.
At the top of the table, you'll see a list of all the apps which are running on your desktop. These are generally programs which you have started yourself. You should recognise them as apps which you're currently using.
The next section, "Background processes," details programs which probably aren't visible on your desktop. These include processes installed by other programs, such as self-update utilities, Windows components and "suspended" Windows Store apps.
At the bottom of the list, you'll find "Windows processes." These are individual components of the Windows operating system. You won't generally need to interact with any of these. They should be left on their own to keep your system running.
You can click the column headings to sort the table by the available fields. This helps you quickly identify apps which are using a lot of RAM, or those which are consuming processor time. Right-clicking a column header allows you to add further fields to the table, such as estimates of power consumption or the complete command which was used to launch the process.
If you need more detailed information, switch over to the "Details" pane. This provides technical details for each process. Again, you can add extra fields and change the sorting using the column headers at the top of the screen.
You can right-click a process to change its attributes. Terminate a process by selecting "End task" – this should work even if the program is unresponsive and you can't use the regular "Close" button.
Finally, we should note that Task Manager only shows apps running on your own desktop. If there are multiple users logged in, you can view their processes by switching to the "Users" tab. This enables you to see if one of their open programs is consuming all the resources of the machine. You'll need to be logged in as an administrator in order to see this information, due to the inherent privacy and security implications of viewing another user's processes.