To put it simply: projects end, while areas continue indefinitely.

However, there are a few other subtleties you should be aware of.
Every project typically falls under an area of responsibility. For example:
Although projects and areas are related, it’s critically important that you distinguish between them. Failing to do so is a root cause of so many frustrations and challenges. Let me illustrate why.
If you have a project (such as writing a book), but you treat it like an ongoing area, without any particular goal or outcome in mind, it will feel aimless and directionless. Likewise, if you have an area (like maintaining a certain weight), but you treat it like it’s just a onetime project, then even if you succeed in losing the extra weight, you’ll likely revert right back afterward because you didn’t put in place long-term habits.
In other words, projects and areas require completely different approaches, mindsets, and tools to be successful. Knowing which ones to use starts with identifying them correctly in the first place.