It concatenates files and prints on the standard output.
e.g.
Merge all files in /var/log with filenames starting with message into a single file ~/all_message_log.
cat /var/log/message* > ~/all_message_log
The command cat is particularly useful when you have no access to any text editor, even vi.
e.g.
When you want to add few lines to an existing files ~/cat/book, you can:
cat >> ~/cat/book
Type what you want to be appended to ~/cat/book and type Ctrl-D (pressing <Ctrl> key and "D" key simultaneously). Show the new ~/cat/book by cat ~/cat/book and check the additional lines at the end of the file.