Assuming Python 3 (in Python 2, this difference is a little less well-defined) - a string is a sequence of characters, ie unicode codepoints; these are an abstract concept, and can't be directly stored on disk. A byte string is a sequence of, unsurprisingly, bytes - things that can be stored on disk. The mapping between them is an encoding - there are quite a lot of these (and infinitely many are possible) - and you need to know which applies in the particular case in order to do the conversion, since a different encoding may map the same bytes to a different string:

>>> b'\xcf\x84o\xcf\x81\xce\xbdo\xcf\x82'.decode('utf-16')
'蓏콯캁澽苏'
>>> b'\xcf\x84o\xcf\x81\xce\xbdo\xcf\x82'.decode('utf-8')
'τoρνoς'

一旦知道使用哪一个，就可以使用字节字符串的.decode()方法从它获得正确的字符串，如上所述。为了完整起见，字符串的.encode()方法采用相反的方式:

>>> 'τoρνoς'.encode('utf-8')
b'\xcf\x84o\xcf\x81\xce\xbdo\xcf\x82'

注意:我将详细阐述我对Python 3的回答，因为Python 2的生命周期已经非常接近了。

Python 3

bytes由8位无符号值的序列组成，而str由表示人类语言文本字符的Unicode码位序列组成。

>>> # bytes
>>> b = b'h\x65llo'
>>> type(b)
<class 'bytes'>
>>> list(b)
[104, 101, 108, 108, 111]
>>> print(b)
b'hello'
>>>
>>> # str
>>> s = 'nai\u0308ve'
>>> type(s)
<class 'str'>
>>> list(s)
['n', 'a', 'i', '̈', 'v', 'e']
>>> print(s)
naïve

尽管bytes和str看起来工作方式相同，但它们的实例彼此不兼容，即bytes和str实例不能与>和+等操作符一起使用。此外，请记住，比较bytes和str实例是否相等，即使用==，即使它们包含完全相同的字符，也将始终计算为False。

>>> # concatenation
>>> b'hi' + b'bye' # this is possible
b'hibye'
>>> 'hi' + 'bye' # this is also possible
'hibye'
>>> b'hi' + 'bye' # this will fail
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't concat str to bytes
>>> 'hi' + b'bye' # this will also fail
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "bytes") to str
>>>
>>> # comparison
>>> b'red' > b'blue' # this is possible
True
>>> 'red'> 'blue' # this is also possible
True
>>> b'red' > 'blue' # you can't compare bytes with str
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '>' not supported between instances of 'bytes' and 'str'
>>> 'red' > b'blue' # you can't compare str with bytes
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '>' not supported between instances of 'str' and 'bytes'
>>> b'blue' == 'red' # equality between str and bytes always evaluates to False
False
>>> b'blue' == 'blue' # equality between str and bytes always evaluates to False
False

处理bytes和str的另一个问题是处理使用open内置函数返回的文件。一方面，如果你想从一个文件中读取或写入二进制数据，总是使用二进制模式打开文件，比如'rb'或'wb'。另一方面，如果要从文件中读取或写入Unicode数据，请注意计算机的默认编码，因此如果需要，可以传递encoding参数以避免意外。

在Python 2中

str由8位值的序列组成，而unicode由unicode字符的序列组成。需要记住的一点是，如果str仅由7位ASCI字符组成，则可以将str和unicode与操作符一起使用。

在Python 2中使用helper函数在str和unicode之间进行转换，在Python 3中使用bytes和str之间进行转换，可能会很有用。

计算机唯一能存储的东西就是字节。

要在计算机中存储任何东西，首先必须对其进行编码，即将其转换为字节。例如:

如果你想要存储音乐，你必须首先使用MP3、WAV等对其进行编码。 如果你想要存储一张图片，你必须先用PNG、JPEG等对它进行编码。 如果想要存储文本，首先必须使用ASCII、UTF-8等对其进行编码。

MP3、WAV、PNG、JPEG、ASCII和UTF-8都是编码的例子。编码是一种以字节表示音频、图像、文本等的格式。

在Python中，一个字节字符串就是:一个字节序列。它不是人类可读的。在底层，所有内容都必须转换为字节字符串，然后才能存储在计算机中。

另一方面，字符串，通常称为“字符串”，是一个字符序列。它是人类可读的。字符串不能直接存储在计算机中，它必须先进行编码(转换为字节字符串)。有多种编码可以将字符串转换为字节字符串，例如ASCII和UTF-8。

'I am a string'.encode('ASCII')

上面的Python代码将使用编码ASCII对字符串'I am a string'进行编码。上述代码的结果将是一个字节字符串。如果你打印它，Python会将它表示为b' i am a string'。然而，请记住，字节字符串不是人类可读的，只是Python在打印它们时将它们从ASCII解码。在Python中，字节字符串由b表示，后面跟着字节字符串的ASCII表示形式。

如果您知道用于编码字节字符串的编码，则可以将字节字符串解码回字符串。

b'I am a string'.decode('ASCII')

上面的代码将返回原始字符串'I am a string'。

编码和解码是反向操作。所有内容都必须在写入磁盘之前进行编码，并且必须在人类读取之前进行解码。

什么是Unicode?：

从根本上说，计算机只是处理数字。它们通过为每个字母和其他字符分配一个数字来存储它们。 ．．．．．． Unicode为每个字符提供了一个唯一的数字，无论什么平台，什么程序，什么语言。

So when a computer represents a string, it finds characters stored in the computer of the string through their unique Unicode number and these figures are stored in memory. But you can't directly write the string to disk or transmit the string on network through their unique Unicode number because these figures are just simple decimal number. You should encode the string to byte string, such as UTF-8. UTF-8 is a character encoding capable of encoding all possible characters and it stores characters as bytes (it looks like this). So the encoded string can be used everywhere because UTF-8 is nearly supported everywhere. When you open a text file encoded in UTF-8 from other systems, your computer will decode it and display characters in it through their unique Unicode number.

当浏览器从网络接收到编码为UTF-8的字符串数据时，它将把数据解码为字符串(假设浏览器采用UTF-8编码)并显示字符串。

在Python 3中，你可以将字符串和字节字符串相互转换:

>>> print('中文'.encode('utf-8'))
b'\xe4\xb8\xad\xe6\x96\x87'
>>> print(b'\xe4\xb8\xad\xe6\x96\x87'.decode('utf-8'))
中文

总而言之，字符串是在计算机上显示给人类阅读的，字节字符串是用于存储到磁盘和数据传输的。

Python语言包括str和bytes作为标准的“内置类型”。换句话说，它们都是类。我认为没有必要去解释为什么Python以这种方式实现。

尽管如此，str和bytes彼此非常相似。两者都有大部分相同的方法。下面的方法对于str类是唯一的:

casefold
encode
format
format_map
isdecimal
isidentifier
isnumeric
isprintable

以下方法对于bytes类是唯一的:

decode
fromhex
hex

Unicode是一种公认的字符二进制表示格式和各种格式(例如，小写/大写、换行和回车)以及其他“东西”(例如，表情符号)的格式。无论是在内存中还是在文件中，计算机存储Unicode表示(一系列位)的能力并不亚于存储ASCII表示(不同的一系列位)或任何其他表示(一系列位)的能力。

为了进行沟通，沟通双方必须就使用何种代表达成一致。

Because Unicode seeks to represent all the possible characters (and other "things") used in inter-human and inter-computer communication, it requires a greater number of bits for the representation of many characters (or things) than other systems of representation that seek to represent a more limited set of characters/things. To "simplify," and perhaps to accommodate historical usage, Unicode representation is almost exclusively converted to some other system of representation (e.g., ASCII) for the purpose of storing characters in files.

并不是说Unicode不能用于在文件中存储字符或通过任何通信通道传输字符。很简单，事实并非如此。

The term "string," is not precisely defined. "String," in its common usage, refers to a set of characters/things. In a computer, those characters may be stored in any one of many different bit-by-bit representations. A "byte string" is a set of characters stored using a representation that uses eight bits (eight bits being referred to as a byte). Since, these days, computers use the Unicode system (characters represented by a variable number of bytes) to store characters in memory, and byte strings (characters represented by single bytes) to store characters to files, a conversion must be used before characters represented in memory will be moved into storage in files.