You can use 'zhidao' on facts, objects and persons. 'renshi' is used for persons. I would limit this word on personal acquaintance. If you have been introduced to someone. For only 'knowing' people (from a magazine, TV, know about someone is doing something) you have to use 'zhidao'. I think.

zhidao means be aware of or learnd renshi means to be familiar with or to recognize

Please look at this example first: wrong sentence：我認識這件事。 wo renshi zhe jian shi. correct sentence：我知道這件事。 wo zhidao zhe jian shi. Reason：You can say『認識人』or『認識到事情的重要性(importance)』，but can not say『認識事情』，so you have to change it to『知道』。 http://big5.hwjyw.com/jxyd/bjxg/200809/t20080927_22409.shtml But this example is not really want to show that 认识 only use to say about you know a person. For example: 我认识这种草药。（I know this kind of herbal medicine） This is a correct sentece, and the object is a thing, not a person 『認識』(认识renshi): Can make sure that this person or this thing is this person or this thing, not other Ex) "我认识他wo renshi ta." （I know him） Please don't say: "我知道他wozhidaota" 『知道』(zhidao): Have the understanding regarding the fact or the truth Ex) "她知道的事情很多。ta zhidao de shiqing hen duo" She knows a lot “我知道她是谁！wo zhidao ta shi shei” I know who is she

"知道zhidao" is usually used to describe objects or people you are not familiar with; "认识renshi" is usually used to describe people, mostly your acquaintances.^_^