In Unix, to send ouput from a process to different file or device or to another process via a pipe, or to have a process read its input from a different file, device or pipe. Some other operating systems have similar facilities.
myprog < myfile
ls > filelist
A pipe redirects the output of one process directly into the input of another
who | wc -l
cat myfile | myprog
Which is more or less equivalent to "myprog < myfile" except that it introduces an extra unnecessary cat process and buffer space for the pipe. Even the "<" is unnecessary with many standard Unix commands since they accept input file names as command line arguments anyway.