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.