help(repr):

repr(...)
    repr(object) -> string
    
    Return the canonical string representation of the object.
    For most object types, eval(repr(object)) == object.

返回对象的典型表示形式（Python解释器所能理解的），后面一句非常重要，对于大多数类型eval(repr(object)) == object

help(str):

class str(basestring)
 |  str(object) -> string
 |  
 |  Return a nice string representation of the object.
 |  If the argument is a string, the return value is the same object.

返回对象的字符串表示（能打印出的人们能理解的）

这样一来，就很好区别了。再来两个例子：

>>> s = "this is a string"
>>> str(s)
'this is a string'
>>> repr(s)
"'this is a string'"

>>> l = [ "l", "i", "s", "t" ]
>>> str(l)
"['l', 'i', 's', 't']"
>>> repr(l)
"['l', 'i', 's', 't']"