expr >> filename writes expr to a file.
Put[, , ... , "filename"] writes a sequence of expressions to a file.
On systems with advanced graphical interfaces, there will usually be graphical tools for saving expressions in files.
Put uses the format type InputForm by default.
Put starts writing output at the beginning of the file. It deletes whatever was previously in the file.
Put inserts a newline (line feed) at the end of its output.
expr >> filename is equivalent to expr >> "filename". The double quotes can be omitted if the file name is of the form specified in Section A.2.7.
It is conventional to use names that end with .m for files containing Mathematica input.
See The Mathematica Book: Section 1.11.1, Section 1.11.10 and Section 2.11.1.
See also: PutAppend, Save, Definition, DumpSave, Export, Get, NotebookPut.