Consider following short Python program:

i= 21;
j= 23;
print (i);
if i>j :
print ('greater')
else :
print 'lesser'

It is giving the error

IndentationError: expected an indented block

What is the cause of this error? (I want to understand it better as I am new to Python.)

You need to indent statements that are intended to be in if-else blocks:

i = 21
j = 23
print (i)if i > j:print('greater')
else:print('lesser')