cat

Concatenate and print (display) the content of files.

Syntax
       cat [-benstuv] [-] [file ...]

Options
     -b      Number the non-blank output lines, starting at 1.

     -n      Number the output lines, starting at 1.

     -s      Squeeze multiple adjacent empty lines, causing the output to be
             single spaced.

     -u      Disable output buffering.

     -v      Displays non-printing characters so they are visible. 
             Control characters print as `^X' for control-X;
             The delete character (octal 0177) prints as `^?' 
             Non-ascii characters (with the high bit set) are printed as `M-' (for meta)
             followed by the character for the low 7 bits.

     -e      Display non-printing characters and display a dollar sign ($) at the
             end of each line.

     -t      Display non-printing characters and display tab characters as ^I at the
             end of each line.

     -       Read from the standard input.

cat exits 0 on success, and >0 if an error occurs.

Examples:

Display a file (this actually works by concatenating the file with STDOUT)
$ cat myfile.txt

Concatenate two files:
$ cat File1.txt File2.txt > union.txt

If you need to combine two files but also eliminate duplicates, this can be done with sort unique:
$ sort -u File1.txt File2.txt > unique_union.txt

Put the contents of a file into a variable
$ my_variable=`cat File3.txt`

“To be nobody but yourself - in a world which is doing its best, night and day, to make you like everybody else - means to fight the hardest battle which any human being can fight, and never stop fighting” - E. E. Cummings

Related:

cat man page - Apple.com
cp - Copy one or more files to another location
mv - Move or rename files or directories
textutil - Manipulate text files in various formats
Stupid Cat tricks - by Mike Chirico