Q:

Cause of expected an indented block error in Python script?

I have been trying to list all the field names of a shapefile but it is showing 'expected an indented block'. 
What should I do?
import arcpy

fieldList = arcpy.ListFields("C:\Users\Mridul\Documents\GIS\Spring 2011\650\streets_dev.shp") 

for field in fieldList: 
print field.name + " is a type of " + field.type + " with a legth of " + field.length

rows = arcpy.UpdateCursor("C:\Users\Mridul\Documents\GIS\Spring 2011\650\streets_dev.shp", "SHAPE_len>400")  

cnt = 0

for row in rows: 
    row.STREETNAME = "OLD " + row.STREETNAME 
    rows.updateRow(row)
    cnt +=  1
    print cnt

del row 
del rows

A:

Your first for loop is missing an indented block.
for field in fieldList: #<----- The next line should be indented
print field.name + " is a type of " + field.type + " with a legth of " + field.length

Like this:
for field in fieldList:
    print field.name + " is a type of " + field.type + " with a legth of " + field.length

Or if the for loop only has a single statement, you can put on the same line.
for field in fieldList: print field.name

Python uses indentation to group statements. For more info, see the python reference.

