zh: /tʂ/ ch: /tʂʰ/ The only difference is aspiration. For the pronunciation of /ʂ/ check this out: http://en.wikipedia.org/wiki/Voiceless_retroflex_sibilant

the gesture and position of tongue of "zh" "ch" and "sh" are exactly the same. so u r right.

the difference is like when you pronounce ㅊandㅈ but then need to also pronounce letter r in english(attention no in korean way) ㅊr으 ---> chī ㅈr으 ---> zhī good luck..

只 吃 是 are exactly the same