SharePoint Online and SharePoint On-prem share the underlying goal of creating web pages for your employees to share documents, interact, submit requests, etc. From an end user perspective, the biggest difference is when updates are rolled out. SharePoint Online is constantly being tweaked and updated with new features and improved performance. There is no latency in getting the updates applied, and there are no major headaches with migration plans to get from an older version to the latest and greatest.
SharePoint on-premises, by contrast, gets patches every few months that need to be manually added to the SharePoint server by your company’s SharePoint team. Also, SharePoint On-premises only gets major updates every few years (e.g. SharePoint 2016, SharePoint 2019, etc), which require a migration plan and many consulting dollars to apply the upgrades.
