/dev/zero, /dev/null

/dev/zero is a special file that provides as many null characters (ASCII NUL, 0x00) as are read from it. One of the typical uses is to provide a character stream for overwriting information. Another might be to generate a clean file of a certain size.

Create a 1MiB file filled with zeroes called 'foobar'
dd if=/dev/zero of=foobar count=1024 bs=1024

Like /dev/null, /dev/zero acts as a source and sink for data. All writes to /dev/zero succeed with no other effects (the same as for /dev/null, although /dev/null is the more commonly used data sink); all reads on /dev/zero return as many NULs as characters requested.

In Unix-like operating systems, /dev/null or the null device is a special file that discards all data written to it (but reports that the write operation succeeded), and provides no data to any process that reads from it (it returns EOF). In Unix programmer jargon, it may also be called the bit bucket or black hole.
The null device is typically used for disposing of unwanted output streams of a process, or as a convenient empty file for input streams.
/dev/null is a special file, not a directory, so one cannot move files into it with the Unix mv command.