1. a person who writes an online journal accessible to users of the Internet.
2. a person who writes an online journal which/that is accessible to users of the Internet.
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Q1: Does #1 mean #2?
Q2: What is an online journal? is it a diary on the Internet? Or an essay?
1 and 2 mean the same thing. #2 is better. Either "which" or "that" is appropriate.
A journal usually does mean a diary, which may be an essay. A diary usually means a recounting of the events of the day ("I had rice for breakfast, went to school, etc., and I'm in love with Jimmy, etc.) A journal may be just that, or a reflection of one's thoughts and ideas (an essay). If it's an online document, it's called a "blog".