cat

Concatenate and print (display) the content of files.

Syntax
      cat [Options] [File]...

Concatenate FILE(s), or standard input, to standard output.

  -A, --show-all           equivalent to -vET

  -b, --number-nonblank    number nonblank output lines

  -e                       equivalent to -vE

  -E, --show-ends          display $ at end of each line

  -n, --number             number all output lines

  -s, --squeeze-blank      never more than one single blank line

  -t                       equivalent to -vT

  -T, --show-tabs          display TAB characters as ^I

  -u                       (ignored)

  -v, --show-nonprinting   use ^ and M- notation, except for LFD and TAB

      --help               display this help and exit

      --version            output version information and exit

With no FILE, or when FILE is -, read standard input.
The cat command can be piped into grep to find specific words in the file:
cat file.txt | grep keyword output.txt

However all modern versions of grep have this built-in. Running a single command/process is more efficient, and so with large files will be noticably faster:
grep keyword file.txt output.txt

grep can also display an entire file, (like cat), by using the grep keyword "." which will match lines with at least 1 character. Alternatively the grep keyword "^" will match the beginning of every line including blank lines.
When grep is used to display multiple files, it will prepend each line of output with the filename:
$ grep . *.txt

Examples: 

Display a file:
$ cat myfile.txt

Display all .txt files:
$ cat *.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:

cp - Copy one or more files to another location.
mv - Move or rename files or directories.
pv - Monitor the progress of data through a pipe.
tac - Concatenate and write files in reverse.
hexdump - View binary file
Stupid Cat tricks - by Mike Chirico.
Equivalent Windows command: TYPE - Display the contents of a text file.