python中的缓冲区意味着什么

在struct的python文档中,使用了单词buffer而没有解释：

http://docs.python.org/library/struct.html

struct.unpack_from(fmt,buffer[,offset=0])

Unpack the buffer
according to the given format. The
result is a tuple even if it contains
exactly one item. The buffer must
contain at least the amount of data
required by the format
(len(buffer[offset:]) must be at least
calcsize(fmt)).

这里的缓冲区是什么意思.字符串是缓冲区还是文件描述符？ “缓冲”必须有哪些方法？