how histogram and multiple bar diagram are in different in appearance as they hav same appearance and how can we decide that for a specific given data we hav to draw a multiple bar diagram and not a histogram.

A multiple bar diagram is drawn when data for a given time period is for various variables simultaneously and a comaprison is to drawn among them for different time periods.
On the other hand, a histogram is draw when data pertaining to a variable is in the form of continuous class intervals. 

For furhter examples on multiple bar diagram and histogram please refer to our studymaterial following the below mentioned path. 
Statistics for Economics- Presentation of Data- Lesson 3 and 4