A pager which displays text files.
Windowed environments like the Windows and Macintosh desktops don't need pagers as much, since they have other methods for viewing files.
Less is not an editor. You can't change the contents of the file you're viewing. Less is not a windowing system. It doesn't have fancy scroll bars or other GUI (graphical user interface) elements. It was designed to work on simple text-only terminals.
Less has been used by thousands of people around the world since its release in 1985.
It's hard to believe some people are still using pagers that don't let you move backwards in the file. Even pagers that do let you move backwards often don't work when viewing "standard input"; that is, data that is being piped into the pager from another program.
Searching and highlighting
Less lets you search for text in the file you're viewing. You can search forwards and backwards. Furthermore, less highlights the text that matches your search, to make it easy to see where the match is.
Less allows you to switch between any number of different files, remembering your position in each file. You can also do a single search which spans all the files you are working with.
Bold and underline
Some text files, notably Unix-style "man pages", use overstriking (backspaces) to simulate underlined and bold text. Less interprets this overstriking and displays the text in real underlined or bold mode.
Less has a large number of fancy features for advanced users. Once you learn the basics, you'll find you can customize less to do almost anything you want it to do. You can change key bindings, set different tab stops, set up filters to view compressed data or other file types, customize the prompt, display line numbers, use "tag" files, and more.
Less is available on many different machines, different system architectures, and different terminals. Pretty much wherever you go, you'll find less. You won't have to learn how to use a different pager on every system you use.