less, more and cat are all file viewers but each has their own particular uses.
cat simply prints the file to the standard output (your screen usually) in one big rush. If it is a simple file with say 10 or 20 lines this is the command to use. more allows you to do the same as cat except it pauses the scrolling every time a screen full has been displayed allowing you to use the space bar to get the next page. less does what more does but allows you to scroll backwards as well as paging forward.
cat
cat is your first choice for viewing a file because it is fast and simple. It is also your best choice if you are piping the output to another command. There are options for this command such as -b -n and -s. To number all lines use -n, to number all lines except blank ones use -b and to skip blank lines use -s. You can combine these and for instance do cat -sn
more
The more command is a bit more useful if you are wanting to page output to look for specific items. However in most cases less is probably more useful. In general you may use more for paging output from other commands especially ones that may contain encoding. If you are wanting to read a static text file then less is most suitable. To stop the output use CTRL+C
less
less is your goto command for file viewing. The basic usage is less filename. You then use the f key to go down a page and the b key to go up a page. This is its main advantage over more, the ability to go back and forward. less also uses a lot of the same commands as vim. So for instance to go to line 100 you would type in 100G where G means go. To quit less use the q key.
The other most useful option of less is the R key. This refreshes the page, this is useful in cases where the file may be changing.
less can also be used instead of tail -f logname.log. To do this use less +F logname.log, you can then CTRL + C to go to editing mode to scroll back and forward.
That is really the basics of using less. It has lots of features though and man less is worth a read in case there are any features that you would find useful in your particular use cases.
Which should I use and when?
The following are some general recommendations about which utility is useful when.
If you are piping the output to another command then use cat in pretty much every case.
If you are not dealing with a file or the output contains ANSI codes then it is generally better to use more. An example of this is if you want to list a directory's contents with color and then page it. ls | more and ls to less don’t work. You need to use ls with the --color=yes option because when you pipe the output it strips it of the ANSI codes. If you try ls --color=yes | more and ls --color=yes | less, you will see that the more output is exactly what you needed whereas the less output has lots of strange characters and no color. You can fix this by using ls –color=yes | less -R however it is much simpler to simply type in | more than | less -R
If you are simply wanting to read a static file that won’t be changing then less is always your best choice.