Viewing file content

cat
Used for viewing files that are not very long; it does not provide any scroll-back.

tac
Used to look at a file backwards, starting with the last line.

less
Used to view larger files because it is a paging program. It pauses at each screen full of text, doesn't load the entire text, therefore provides scroll-back capabilities. Also, lets you search and navigate within the file.
NOTE: Use / to search for a pattern in the forward direction and ? for a pattern in the backward direction. An older program named more is still used, but has fewer capabilities: "less is more".

more
Used to view larger files. It pauses at each screen full of text, It loads the entire file and allows to goup and down through paging.

tail
Used to print the last 10 lines of a file by default. You can change the number of lines by doing -n 15 or just -15 if you wanted to look at the last 15 lines instead of the default.

head
The opposite of tail; by default, it prints the first 10 lines of a file.