Previously known as Magento Community Edition, Magento Open Source is the ecommerce platform. Adobe which acquired Magento in 2018 has confirmed the open source edition will remain part of their plans going forward.
In web development open source means that software can be used on almost any platform and can easily updated, changed and redistributed as required. When you choose the open source option you will have the freedom to update and extend your store system without going back to the original developers.
Magento Commerce is the paid-for premium version of the Magento ecommerce platform. This version of the platform is managed by Adobe and they will take care of your hosting and but this also include a yearly minimum fee of US$22,000 for their services.
With Magento even the free, open source version includes enterprise level features such as PCI compliant payment solutions, marketing and promotional tools, full featured CMS and shipping/CRM integrations.
Choosing the right ecommerce platform is always the first decision when starting out on the journey to an ecommerce website if you need help let me know!