Ni hao is said to your peers or to the younger like your sisters and children.But Nin Hao is used when you greet the elder like your parents and grandparents.They have the same meaning and both used when you greet others.

ni你=تو nin您=شما

nihao and ninhao are both for greeting...but ninhao is showing ur most respect, to the people such as ur seniors....while ni hao is not so much formal..u can say it to ur friend ...

意思上没有太大区别，您是后造字，表敬意。