There are so many potential workflows, analysis settings, and datasets within the broad field of GIS that no out-of-the-box software could contain the tools for every scenario. Fortunately, QGIS has been developed with a plugin architecture. Plugins are add-ons to QGIS that provide additional functionality. Some are written by the core QGIS development team and others are written by QGIS users.

You can explore the QGIS plugin ecosystem by navigating to Plugins | Manage and Install Plugins. This opens the Plugins Manager window (shown in figure below) that will allow you to browse all plugins, those that are installed, and those that are not installed, and adjust the settings. If there are installed plugins with available upgrades, there will also be an Upgradable option. The search bar can be used to enter search terms and find available plugins related to the topic. This is the first place to look if there's a tool or extra type of functionality that you need! To install a plugin, simply select it and click on the Install Plugin button. Installed plugins can be toggled on and off by checking the box next to each.

You will be notified by a link at the bottom of the QGIS Desktop application if there are updates available for your installed plugins. Clicking on the link will open the Plugins Manager window, where the Upgrades tab will allow you to install all or some of the available updates. Plugins themselves may show up as individual buttons, toolbars, or as items under the appropriate menu, such as Plugins, Vector, Raster, Database, Web, or Processing.