Plugins extend Craft with extra functionality — search, redirects, SEO, calendars, and more. They’re installed and configured by development.
Some plugins provide functionality the site requires per its specifications, and some save significant development time or do things beyond what’s practical to build from scratch. Paid plugins are chosen when they’re the most cost-effective way to meet a requirement.
Many high-quality plugins are maintained by the Craft community at no cost. Whether a plugin is free is up to its author.
Some commercial plugins include an optional yearly renewal that funds ongoing development and updates. Renewing keeps the plugin eligible for the latest updates; letting it lapse usually means the installed version keeps working but stops receiving updates.
Generally no. Plugins are set up by development. A few have editor-facing screens (for example redirects and search), which are covered on their own Help pages.
The exact set evolves over time; current highlights include:
Note: During the upgrade to Craft 5, two plugins that earlier versions of this manual referenced — Blockonomicon and Reliquary — were removed. Their roles are now handled natively: page modules are built with Craft’s nested entries (see the “Rebar Module System” page), and search is handled by the SiteSearch module (see the “SiteSearch” page).