A closed model is accessed through an app or API without releasing the model weights. An open-weight model releases model weights so people can run it, but the license may still limit use. A truly open-source AI project also provides code, data details, and licensing that fit open-source expectations.
In practice
Open-weight models are popular for local AI, research, and customization. Closed models are often easier to use at high quality because the provider manages hosting and updates.
What to watch
Do not assume "open" means unrestricted. Always check the license, commercial terms, attribution rules, and acceptable use policy.