OpenRead["file"] opens a file to read data from, and returns an InputStream object.
OpenRead prepares to read from a file, starting at the beginning of the file.
On systems that support pipes, OpenRead["!command"] runs the external program specified by command, and opens a pipe to get input from it.
If OpenRead does not succeed in opening a particular file or pipe, it generates a message, and returns $Failed.
OpenRead resolves file names according to the procedure described in Section A.6.1.
The function ReadList automatically opens files or pipes that it needs.
OpenRead returns InputStream["name", n], where name is the full name of a file or command, and n is a serial number that is unique across all streams opened in the current Mathematica session.
Setting the option DOSTextFormat->True causes all input to be treated as coming from a text-mode file on an MS-DOS or related system. This means that \r\n pairs are interpreted as single newlines, and Z is interpreted as EndOfFile.
See The Mathematica Book: Section 2.11.8.
See also: Close, Read, ReadList, Streams, LinkCreate.