Plasmoids are widgets for the KDE desktop. You can add plasmoids by clicking the plasma toolbox icon located at the top right of the desktop or right-hand edge of the panel and then clicking Add Widgets.... You can also add plasmoids by right-clicking on the desktop and panel and clicking Add Widgets....

When you install the KDE desktop, a number of plasmoids are installed on your system by default, although most of them are not visible until you add them to your desktop or panel. These include various clocks, calendars, small games, and widgets that present you with information about the status of your computer hardware or about multimedia files as you play them. When you click the Add Widgets... menu option, the plasma toolbox presents you with a list of the widgets currently available to you, along with short descriptions of each one. The plasma toolbox also gives you the option to Get New Widgets either by downloading from KDE-Look.org, or by installing ones that you have previously downloaded and saved to your computer.