ChatGPT is the common name of AI operated chat software created by Open AI.
ChatGPT 3.5 is the current, free, version that is available for all users to explore and test out the abilities of an AI prompt system.
ChatGPT Plus and ChatGPT Enterprise are the most recent options put out by Open AI. These options are subscription based and do have associated fees. There are various benefits to these options, though Plus currently uses the same platform as 3.5.
The main difference is in its availability. ChatGPT is accessible via an app or desktop and has a free version that allows exploration without the need to purchase software or a subscription.