Many younger people, might not know lpr command, historically it was heavily used for printing in the early GNU / Linux days.
lpr ships the text to be printed to the printer which is physically attached on LPT (Line Print Terminal) parallel port . Those who lived the DOS era surely know in those "ancient" days, everyone who wanted to print has to use the LPT parallel port
Present time, everyone knows there is almost no modern printer that is attached to the PC via LPT port but rather the USB port is used for communication between the printer the computer.
Nevertheless The USB printers on Linux are managed by CUPS, the lpr command is still functional shipping the text to be printed via CUPS (cups-lpd daemon).
Before cups-lpd was introduced the service managing the print jobs was lpd
Hence lpr is still functional.
To print a plain text file of one page with lpr on Linux:
linux:~# cat text-file-to-print.txt | lpr
For multiple printers to switch between multiple printers there is the PRINTER shell variable:
linux:~# export PRINTER=printer-Name-and-Type
To print a really long text file (a book in TXT) the pr command comes handy. As you can read in the cmd manual pr – converts text files for printing
Lets say you would like to have a 60 lines of text per printed page, the cmd to issue is:
linux:~# pr -l60 text-file-to-print.txt | lrp
All queued printing jobs can be reviewed with the lpq, if you have a printer attached try:
linux:~# lpq
lp is ready and printing
Rank Owner Job Files Total Size
active hipo 1 text-file-to-print.txt 62045 bytes
Since some years it is pretty rare for people to use lpq, since most of the parallel printing is managed by CUPS server, what most people use nowdays to check the printer queue is lpstat : e.g.
linux:~# lpstat
...
Printing status and all things related to queued jobs for printing gets logged in /var/log/lpr.log
There is even more simplistic way to print directly to the printer (if the printer is attached via a LPT port) through the kernel /dev/lp, for example:
linux:~# cat text-file-to-print.txt >> /dev/lp
For more than one printer attached the naming of /dev/lp, might probably be /dev/lp0, /dev/lp1 etc.
The lprm command also exists in case if you would like to cancel a printjob in the queue. Lets say I want to cancel a job in the queue with Job ID 5:
linux:~# lrpm 5
...
To cancel a current running job in the middle the /usr/bin/cancel command exists.
An interesting historic fact is that nowdays opening lpr, lpq or any of the other tools for simple text mode printing one sees on top of the page Apple Inc.
Lets clear this up CUPS (Common Unix Printing System) (open source) printing platform is not owned by Apple, since it is licensed under GPL2 and LGPL. The reason why the Apple Inc. shows up in man pages is because in year 2007, the founder of CUPS printing server Michael Sweet hired him to work for Apple Inc. "purchasing" the CUPS source. However as we know they did not really purchased the code, because the code was already belonging to the community (licensed under GPL2). Apple however as a marketing trick used the fact that Sweet worked for them and as probably as a matter of marketing asked him to place the Apple Inc. in the copyright source and manual areas. Obviously this is not true, since Apple Inc. does not hold copyright for CUPS as CUPS can be copied by anyone (its open source) 
Most of the people will never print using this commands, since printing is now, ages ahead, anyways for simple people (like me), who just need to print a text with no special fonts or graphics text printing is just great.
Text printing is also a good learning experience for Linux novices and is good to be known just as a piece of UNIX history.
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