is defined - The variable has been defined, and set to any value (or null).
is not null - The variable has been defined, and is specifically not null.
In PHP terms, it's like the difference between is_null() vs. checking whether the variable exists using get_defined_vars() (see this Stack Overflow thread for a little more clarity)
Try this experiment... Without setting myVar, put this in your template:
{{ (myVar is defined) }}
Since that equates to false, nothing will appear in your template. Now change it to this:
{{ (myVar is not null) }}
That will actually throw an error, since myVar was never defined.