They can mean the same thing

Like keeping a daily log of your events.

But journal can be more broad... Like writing a journal of scientific research. It didn't necessarily need to be daily or related to dates or even in chronological order. But it usually is about one subject or concept.

Like someone who loves reading might keep a journal of summaries of the books they've read.

While diary usually means some sort of record over time. Diary comes from the Latin word for days. A journal can be this too but it can be so much more.

To explain it like you are five well there are different types of fat cells Visceral fat produces more toxic substances than subcutaneous fat, so it is more dangerous. Even in thin people, having visceral fat carries a range of health risks. It is a type of fat that wraps around your internal organs. Unfortunately being obese means you most likely have that type of fat as well.