多看看帮助 : )
TFileStream enables applications to read from and write to a file on disk.
TFileStream 使应用程序能够读写硬盘文件。
Unit
classes
Description
Use TFileStream to access the information in disk files. TFileStream will open a named file and provide methods to read from or write to it. If an application already has a handle to the file, opened in the appropriate mode, use THandleStream instead.
使用TFileStream 去访问磁盘文件的信息，Tfilestream打开文件，并提供方法读和写。
如果应用程序已经有文件handle，则使用Thandlestream。
TStringStream provides file-like access to information stored as a long string.
TStringStream 提供像访问文件的方式访问存储的字符串信息.
Unit
classes
Description
Use TStringStream to store data as a long string enhanced with I/O capabilities. TStringStream is useful as an intermediary object that can hold text as well as read it from or write it to another storage medium. TStringStream provides a mechanism for manipulating text that is obtained from a less accessible medium.
Tstringstream提供一个机制访问，维护长串。
如果操作字符串应该使用tstringstream，因为它对字符串操作优化。
如果操作文件型的读写则使用tfilestream。
这是我的个人理解，不正确的地方请更正。
