There are several approaches you can take when reading from and writing to files:

• The recommended approach for working with files is to use file streams. File streams are instances of the TFileStream class used to access information in disk files. File streams are a portable and high-level approach to file I/O. Because file streams make the file handle available, this approach can be combined with the next one. The Using file streams discusses TFileStream in detail.
• You can work with files using a handle-based approach. File handles are provided by the operating system when you create or open a file to work with its contents. The SysUtils unit defines a number of file-handling routines that work with files using file handles. On Windows, these are typically wrappers around Windows API functions. Because the VCL/RTL functions can use the Delphi language syntax, and occasionally provide default parameter values, they are a convenient interface to the Windows API. Furthermore, there are corresponding versions on Linux, so you can use these routines in cross-platform applications. To use a handle-based approach, you first open a file using the FileOpen function or create a new file using the FileCreate function. Once you have the handle, use handle-based routines to work with its contents (write a line, read text, and so on).
• The System unit defines a number of file I/O routines that work with file variables, usually of the format "F: Text:" or "F: File:" File variables can have one of three types: typed, text, and untyped. A number of file-handling routines, such as AssignPrn and writeln, use them. The use of file variables is deprecated, and these file types are supported only for backward compatibility. They are incompatible with Windows file handles.