Before you understand what is a Histogram, you are advised to read:
What is Grouped Frequency Table ?
The graphical representation of data of class interval is called as Histogram. Or in other words, A Histogram is a bar graphs that shows data in intervals. Or we can also say that: A Graph which represents grouped frequency table; is known as Histogram
Since there is no gap between the classintervals, there is no gaps between the bars of histogram too and height of bar represents the frequency of class interval.
Example : In a class of 30 students, following numbers represents weight of these students: 30, 31, 51, 35, 40, 39, 42, 43, 36, 49, 44, 33, 37, 47, 52, 38, 32, 50, 39, 40, 45, 46, 53, 35, 31, 41, 51, 33, 46, 53
Put the data in groups and make a Grouped Frequency Distribution Table (as shown below):
Groups (Weight in Kg)  Frequency (No. of Students) 

30  35  6  35  40  7  40  45  6  45  50  5  50  55  6  Total  30 

