This something of a conundrum that can be best answered in the same enquiring tone as the question was asked.
Generally speaking a 'zen' garden is reckoned to be one that has few, if any, plants but has carefully arranged stones and raked gravel it's purpose being to aid meditation.
A 'Japanese' garden has plants and rocks and water it's purpose being to celebrate nature.
Both are Japanese in origin and both may, or may not, have religious connotations.